When the Error API was created, we adopted the (unwritten) rule to
return void when the function returns no useful value on success,
unlike GError, which recommends to return true on success and false on
error then.

When a function returns a distinct error value, say false, a checked
call that passes the error up looks like

    if (!frobnicate(..., errp)) {
        handle the error...
    }

When it returns void, we need

    Error *err = NULL;

    frobnicate(..., &err);
    if (err) {
        handle the error...
        error_propagate(errp, err);
    }

Not only is this more verbose, it also creates an Error object even
when @errp is null, &error_abort or &error_fatal.

People got tired of the additional boilerplate, and started to ignore
the unwritten rule.  The result is confusion among developers about
the preferred usage.

This series adopts the GError rule (in writing), and updates a
substantial amount of code to honor the rule.  Cuts the number of
error_propagate() calls nearly by half.  The diffstat speaks for
itself.

Also available from my public repository
https://repo.or.cz/qemu/armbru.git on branch error-smooth.

v3:
* Rebased
* Fix patch ordering: old PATCH 19 becomes PATCH 37
* PATCH 03+13+25+27: Avoid long lines in Coccinelle script [Eric]
* PATCH 14: Fix commit message typo [Eric]
* PATCH 16: Unbreak opts_start_list(), qapi_clone_start_list(),
  qapi_clone_start_alternate() [Vladimir]
* PATCH 24: Unbreak object_set_link_property() [Vladimir]
* PATCH 25+33+35: Move unrelated hunks from 25 to 33 and 35
  [Vladimir], tweak line breaks
* PATCH 32: Have commit message point out the unnecessary
  error_propagate() will be eliminated shortly [Eric]
* PATCH 33: Fix commit message typo [Eric]
* PATCH 35: Delete comment along with the assertion [Eric]
* Left for later: followup to fix nearby typos and such [Eric]

v2:
* Rebased
* Coccinelle scripts reworked, patches sliced and diced for
  reviewability:
  Old PATCH 03+17+23-24+35+38-39+42 split into "Use returned bool to
  check for failure" parts [PATCH 03+13+17-18+25+28-29+42], and
  "Eliminate error_propagate()" parts.  The latter combined with old
  PATCH 06-07+18-19+29+43 are now [PATCH 33-37].  The end result is
  almost identical.  Some R-bys dropped; sorry!
* Drop variables as they become unused [Vladimir]
* PATCH 01: Comment typos fixed [Greg]
* PATCH 09: Simplify further by cutting out variables [Eric]
* PATCH 11: opt_set() renamed to opt_validate(), redundant parameter
  dropped [Vladimir], R-by kept
* PATCH 16: Comment typos fixed, indentation tidied up [Eric]
* PATCH 23: Assertion dropped [Eric], incorrect hunk backed out
* PATCH 26: Regenerated after rebase; note additional Coccinelle
  hiccup in the commit message
* PATCH 27: Indentation tiedied up [Eric]
* Left for later: followup to fix nearby typos and such [Eric]

Markus Armbruster (44):
  error: Improve examples in error.h's big comment
  error: Document Error API usage rules
  qdev: Use returned bool to check for qdev_realize() etc. failure
  macio: Tidy up error handling in macio_newworld_realize()
  virtio-crypto-pci: Tidy up virtio_crypto_pci_realize()
  qemu-option: Check return value instead of @err where convenient
  qemu-option: Make uses of find_desc_by_name() more similar
  qemu-option: Factor out helper find_default_by_name()
  qemu-option: Simplify around find_default_by_name()
  qemu-option: Factor out helper opt_create()
  qemu-option: Replace opt_set() by cleaner opt_validate()
  qemu-option: Make functions taking Error ** return bool, not void
  qemu-option: Use returned bool to check for failure
  block: Avoid error accumulation in bdrv_img_create()
  hmp: Eliminate a variable in hmp_migrate_set_parameter()
  qapi: Make visitor functions taking Error ** return bool, not void
  qapi: Use returned bool to check for failure, Coccinelle part
  qapi: Use returned bool to check for failure, manual part
  s390x/pci: Fix harmless mistake in zpci's property fid's setter
  qom: Use error_reportf_err() instead of g_printerr() in examples
  qom: Rename qdev_get_type() to object_get_type()
  qom: Crash more nicely on object_property_get_link() failure
  qom: Don't handle impossible object_property_get_link() failure
  qom: Use return values to check for error where that's simpler
  qom: Put name parameter before value / visitor parameter
  qom: Make functions taking Error ** return bool, not void
  qom: Use returned bool to check for failure, Coccinelle part
  qom: Use returned bool to check for failure, manual part
  qom: Make functions taking Error ** return bool, not 0/-1
  qdev: Make functions taking Error ** return bool, not void
  qdev: Use returned bool to check for failure, Coccinelle part
  error: Avoid unnecessary error_propagate() after error_setg()
  error: Eliminate error_propagate() with Coccinelle, part 1
  error: Eliminate error_propagate() with Coccinelle, part 2
  error: Eliminate error_propagate() manually
  error: Reduce unnecessary error propagation
  block/parallels: Simplify parallels_open() after previous commit
  qapi: Smooth another visitor error checking pattern
  qapi: Smooth visitor error checking in generated code
  qapi: Purge error_propagate() from QAPI core
  error: Avoid error_propagate() after migrate_add_blocker()
  qemu-img: Ignore Error objects where the return value suffices
  qdev: Ignore Error objects where the return value suffices
  hmp: Ignore Error objects where the return value suffices

 docs/devel/qapi-code-gen.txt             | 103 ++++-----
 include/hw/audio/pcspk.h                 |   2 +-
 include/hw/qdev-properties.h             |   4 +-
 include/qapi/clone-visitor.h             |   8 +-
 include/qapi/error.h                     |  45 +++-
 include/qapi/visitor-impl.h              |  26 +--
 include/qapi/visitor.h                   | 102 +++++----
 include/qemu/option.h                    |  16 +-
 include/qom/object.h                     | 104 +++++----
 include/qom/object_interfaces.h          |  12 +-
 include/qom/qom-qobject.h                |   9 +-
 accel/kvm/kvm-all.c                      |  55 +++--
 accel/tcg/tcg-all.c                      |   5 +-
 audio/audio_legacy.c                     |  15 +-
 backends/cryptodev-vhost-user.c          |   3 +-
 backends/cryptodev.c                     |  16 +-
 backends/hostmem-file.c                  |  22 +-
 backends/hostmem-memfd.c                 |  18 +-
 backends/hostmem.c                       |  33 ++-
 backends/rng.c                           |   2 +-
 backends/tpm/tpm_util.c                  |   5 +-
 block.c                                  |  21 +-
 block/blkdebug.c                         |   9 +-
 block/blklogwrites.c                     |   4 +-
 block/blkverify.c                        |   4 +-
 block/crypto.c                           |   5 +-
 block/curl.c                             |   5 +-
 block/file-posix.c                       |  16 +-
 block/file-win32.c                       |   8 +-
 block/gluster.c                          |  17 +-
 block/iscsi.c                            |   4 +-
 block/nbd.c                              |  10 +-
 block/nfs.c                              |   7 +-
 block/parallels.c                        |  29 +--
 block/qcow.c                             |  16 +-
 block/qcow2.c                            |  21 +-
 block/qed.c                              |  10 +-
 block/quorum.c                           |  19 +-
 block/raw-format.c                       |   5 +-
 block/rbd.c                              |   7 +-
 block/replication.c                      |  19 +-
 block/sheepdog.c                         |  16 +-
 block/ssh.c                              |  11 +-
 block/throttle-groups.c                  |  31 +--
 block/throttle.c                         |   5 +-
 block/vdi.c                              |  13 +-
 block/vhdx.c                             |  15 +-
 block/vmdk.c                             |  13 +-
 block/vpc.c                              |  19 +-
 block/vvfat.c                            |  10 +-
 block/vxhs.c                             |  15 +-
 blockdev.c                               |  40 ++--
 bootdevice.c                             |  13 +-
 chardev/char.c                           |   6 +-
 contrib/ivshmem-server/main.c            |   4 +-
 crypto/secret.c                          |   2 +-
 crypto/secret_keyring.c                  |   2 +-
 crypto/tlscredsanon.c                    |   2 +-
 crypto/tlscredspsk.c                     |   2 +-
 crypto/tlscredsx509.c                    |   2 +-
 dump/dump.c                              |   7 +-
 hw/acpi/core.c                           |  19 +-
 hw/acpi/cpu_hotplug.c                    |   4 +-
 hw/acpi/ich9.c                           |   2 +-
 hw/acpi/piix4.c                          |   2 +-
 hw/arm/allwinner-a10.c                   |  27 +--
 hw/arm/armsse.c                          | 208 ++++++------------
 hw/arm/armv7m.c                          |  47 ++--
 hw/arm/aspeed.c                          |  24 +--
 hw/arm/aspeed_ast2600.c                  | 124 ++++-------
 hw/arm/aspeed_soc.c                      |  85 +++-----
 hw/arm/bcm2835_peripherals.c             |  81 ++-----
 hw/arm/bcm2836.c                         |  35 +--
 hw/arm/cubieboard.c                      |  14 +-
 hw/arm/digic.c                           |  18 +-
 hw/arm/digic_boards.c                    |   3 +-
 hw/arm/exynos4210.c                      |  13 +-
 hw/arm/fsl-imx25.c                       |  58 ++---
 hw/arm/fsl-imx31.c                       |  34 +--
 hw/arm/fsl-imx6.c                        |  85 +++-----
 hw/arm/fsl-imx6ul.c                      |  24 +--
 hw/arm/fsl-imx7.c                        |  31 ++-
 hw/arm/highbank.c                        |  12 +-
 hw/arm/integratorcp.c                    |   2 +-
 hw/arm/microbit.c                        |   4 +-
 hw/arm/mps2-tz.c                         |  31 ++-
 hw/arm/mps2.c                            |  12 +-
 hw/arm/msf2-soc.c                        |  29 +--
 hw/arm/musca.c                           |  18 +-
 hw/arm/musicpal.c                        |   4 +-
 hw/arm/nrf51_soc.c                       |  36 +---
 hw/arm/orangepi.c                        |  13 +-
 hw/arm/raspi.c                           |   2 +-
 hw/arm/realview.c                        |   6 +-
 hw/arm/sbsa-ref.c                        |  16 +-
 hw/arm/stellaris.c                       |   4 +-
 hw/arm/stm32f205_soc.c                   |  37 +---
 hw/arm/stm32f405_soc.c                   |  48 ++---
 hw/arm/versatilepb.c                     |   4 +-
 hw/arm/vexpress.c                        |   8 +-
 hw/arm/virt.c                            |  44 ++--
 hw/arm/xilinx_zynq.c                     |   6 +-
 hw/arm/xlnx-versal-virt.c                |   8 +-
 hw/arm/xlnx-versal.c                     |  30 ++-
 hw/arm/xlnx-zcu102.c                     |   8 +-
 hw/arm/xlnx-zynqmp.c                     | 117 ++++------
 hw/block/fdc.c                           |  12 +-
 hw/block/xen-block.c                     |  30 +--
 hw/char/serial-pci-multi.c               |   5 +-
 hw/char/serial-pci.c                     |   5 +-
 hw/char/serial.c                         |  10 +-
 hw/core/bus.c                            |  12 +-
 hw/core/cpu.c                            |   3 +-
 hw/core/machine.c                        |   5 +-
 hw/core/numa.c                           |  55 ++---
 hw/core/platform-bus.c                   |   6 +-
 hw/core/qdev-properties-system.c         |  32 +--
 hw/core/qdev-properties.c                |  95 +++------
 hw/core/qdev.c                           |  16 +-
 hw/cpu/a15mpcore.c                       |   5 +-
 hw/cpu/a9mpcore.c                        |  21 +-
 hw/cpu/arm11mpcore.c                     |  17 +-
 hw/cpu/core.c                            |  10 +-
 hw/cpu/realview_mpcore.c                 |   9 +-
 hw/display/bcm2835_fb.c                  |   8 +-
 hw/display/virtio-gpu-base.c             |   5 +-
 hw/display/virtio-gpu-pci.c              |  11 +-
 hw/display/virtio-vga.c                  |  10 +-
 hw/dma/bcm2835_dma.c                     |   9 +-
 hw/dma/sparc32_dma.c                     |   6 +-
 hw/dma/xilinx_axidma.c                   |   4 +-
 hw/gpio/aspeed_gpio.c                    |   5 +-
 hw/gpio/bcm2835_gpio.c                   |  15 +-
 hw/hyperv/vmbus.c                        |   5 +-
 hw/i386/pc.c                             |  48 ++---
 hw/i386/pc_piix.c                        |   4 +-
 hw/i386/pc_q35.c                         |  28 +--
 hw/i386/x86.c                            |   7 +-
 hw/ide/qdev.c                            |   7 +-
 hw/intc/apic_common.c                    |   5 +-
 hw/intc/arm_gic_kvm.c                    |   4 +-
 hw/intc/arm_gicv3_its_kvm.c              |   5 +-
 hw/intc/arm_gicv3_kvm.c                  |   4 +-
 hw/intc/armv7m_nvic.c                    |   9 +-
 hw/intc/nios2_iic.c                      |   8 +-
 hw/intc/pnv_xive.c                       |  17 +-
 hw/intc/realview_gic.c                   |   5 +-
 hw/intc/spapr_xive.c                     |  17 +-
 hw/intc/xics.c                           |   9 +-
 hw/intc/xics_kvm.c                       |   4 +-
 hw/intc/xive.c                           |  17 +-
 hw/isa/piix4.c                           |   5 +-
 hw/m68k/q800.c                           |   4 +-
 hw/mem/nvdimm.c                          |  30 +--
 hw/mem/pc-dimm.c                         |  18 +-
 hw/microblaze/petalogix_ml605_mmu.c      |  24 +--
 hw/microblaze/petalogix_s3adsp1800_mmu.c |   2 +-
 hw/microblaze/xlnx-zynqmp-pmu.c          |  39 ++--
 hw/mips/boston.c                         |   4 +-
 hw/mips/cps.c                            |  41 ++--
 hw/mips/jazz.c                           |   4 +-
 hw/mips/malta.c                          |   4 +-
 hw/misc/aspeed_sdmc.c                    |   8 +-
 hw/misc/bcm2835_mbox.c                   |   9 +-
 hw/misc/bcm2835_property.c               |  17 +-
 hw/misc/iotkit-sysctl.c                  |   2 +-
 hw/misc/ivshmem.c                        |   4 +-
 hw/misc/macio/cuda.c                     |   5 +-
 hw/misc/macio/macio.c                    |  35 ++-
 hw/misc/macio/pmu.c                      |   5 +-
 hw/misc/pca9552.c                        |   5 +-
 hw/misc/tmp105.c                         |   5 +-
 hw/misc/tmp421.c                         |   5 +-
 hw/net/ne2000-isa.c                      |   7 +-
 hw/net/virtio-net.c                      |   7 +-
 hw/net/xilinx_axienet.c                  |   4 +-
 hw/pci-host/pnv_phb3.c                   |  33 ++-
 hw/pci-host/pnv_phb4.c                   |   9 +-
 hw/pci-host/pnv_phb4_pec.c               |   9 +-
 hw/pci-host/prep.c                       |   4 +-
 hw/ppc/e500.c                            |   5 +-
 hw/ppc/mac_newworld.c                    |  10 +-
 hw/ppc/mac_oldworld.c                    |   4 +-
 hw/ppc/pnv.c                             | 171 +++++++--------
 hw/ppc/pnv_core.c                        |   4 +-
 hw/ppc/pnv_psi.c                         |  22 +-
 hw/ppc/rs6000_mc.c                       |   9 +-
 hw/ppc/spapr.c                           |  77 +++----
 hw/ppc/spapr_caps.c                      |  15 +-
 hw/ppc/spapr_cpu_core.c                  |  15 +-
 hw/ppc/spapr_drc.c                       |  16 +-
 hw/ppc/spapr_hcall.c                     |   3 +-
 hw/ppc/spapr_irq.c                       |  11 +-
 hw/ppc/spapr_pci.c                       |  16 +-
 hw/ppc/spapr_pci_nvlink2.c               |   3 +-
 hw/riscv/opentitan.c                     |  13 +-
 hw/riscv/sifive_e.c                      |  10 +-
 hw/riscv/sifive_u.c                      |  11 +-
 hw/riscv/spike.c                         |   4 +-
 hw/riscv/virt.c                          |   4 +-
 hw/rx/rx-gdbsim.c                        |  12 +-
 hw/s390x/css.c                           |   5 +-
 hw/s390x/event-facility.c                |  13 +-
 hw/s390x/ipl.c                           |  27 ++-
 hw/s390x/s390-pci-bus.c                  |  14 +-
 hw/s390x/s390-skeys.c                    |   2 +-
 hw/s390x/s390-stattrib.c                 |   2 +-
 hw/s390x/s390-virtio-ccw.c               |  20 +-
 hw/s390x/sclp.c                          |  13 +-
 hw/s390x/virtio-ccw-crypto.c             |  10 +-
 hw/s390x/virtio-ccw-rng.c                |   8 +-
 hw/scsi/scsi-bus.c                       |  15 +-
 hw/scsi/vhost-scsi.c                     |   4 +-
 hw/sd/aspeed_sdhci.c                     |  15 +-
 hw/sd/sd.c                               |   3 +-
 hw/sd/ssi-sd.c                           |  11 +-
 hw/smbios/smbios.c                       |  33 +--
 hw/sparc/sun4m.c                         |   2 +-
 hw/sparc64/sun4u.c                       |   2 +-
 hw/usb/bus.c                             |   7 +-
 hw/usb/dev-storage.c                     |   9 +-
 hw/usb/hcd-dwc2.c                        |   9 +-
 hw/vfio/pci-quirks.c                     |   5 +-
 hw/vfio/pci.c                            |  10 +-
 hw/virtio/virtio-balloon.c               |  17 +-
 hw/virtio/virtio-crypto-pci.c            |   9 +-
 hw/virtio/virtio-iommu-pci.c             |   4 +-
 hw/virtio/virtio-pmem-pci.c              |   2 +-
 hw/virtio/virtio-rng-pci.c               |   8 +-
 hw/virtio/virtio-rng.c                   |  11 +-
 hw/xen/xen_pt_config_init.c              |   3 +-
 iothread.c                               |  18 +-
 linux-user/syscall.c                     |   2 +-
 monitor/hmp-cmds.c                       |  11 +-
 monitor/monitor.c                        |  21 +-
 net/colo-compare.c                       |  26 +--
 net/dump.c                               |  13 +-
 net/filter-buffer.c                      |  13 +-
 net/filter.c                             |   2 +-
 net/net.c                                |  10 +-
 net/tap.c                                |   6 +-
 qapi/opts-visitor.c                      |  66 +++---
 qapi/qapi-clone-visitor.c                |  69 +++---
 qapi/qapi-dealloc-visitor.c              |  27 ++-
 qapi/qapi-visit-core.c                   | 197 +++++++++--------
 qapi/qobject-input-visitor.c             | 109 ++++++----
 qapi/qobject-output-visitor.c            |  27 ++-
 qapi/string-input-visitor.c              |  67 +++---
 qapi/string-output-visitor.c             |  32 +--
 qdev-monitor.c                           |  39 ++--
 qemu-img.c                               |  23 +-
 qga/commands-posix.c                     |   4 +-
 qga/commands-win32.c                     |  22 +-
 qom/object.c                             | 244 ++++++++++-----------
 qom/object_interfaces.c                  |  30 +--
 qom/qom-hmp-cmds.c                       |   2 +-
 qom/qom-qmp-cmds.c                       |   2 +-
 qom/qom-qobject.c                        |  14 +-
 softmmu/vl.c                             |  17 +-
 target/arm/cpu64.c                       |  15 +-
 target/arm/monitor.c                     |   7 +-
 target/i386/cpu.c                        |  98 +++------
 target/ppc/compat.c                      |   5 +-
 target/ppc/translate_init.inc.c          |   2 +-
 target/s390x/cpu_models.c                |  17 +-
 target/sparc/cpu.c                       |   5 +-
 tpm.c                                    |   5 +-
 ui/console.c                             |   4 +-
 ui/vnc.c                                 |   2 +-
 util/main-loop.c                         |   4 +-
 util/qemu-config.c                       |  25 +--
 util/qemu-option.c                       | 259 ++++++++++++-----------
 scripts/qapi/commands.py                 |  22 +-
 scripts/qapi/visit.py                    | 107 ++++------
 274 files changed, 2362 insertions(+), 3534 deletions(-)

-- 
2.26.2

diff between v2 rebased and v3, with hunks that change only whitespace
dropped:

diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
index 6d8f4b6928..7781c23a42 100644
--- a/qapi/opts-visitor.c
+++ b/qapi/opts-visitor.c
@@ -235,12 +235,12 @@ opts_start_list(Visitor *v, const char *name, GenericList **list, size_t size,
     /* we don't support visits without a list */
     assert(list);
     ov->repeated_opts = lookup_distinct(ov, name, errp);
-    if (ov->repeated_opts) {
-        ov->list_mode = LM_IN_PROGRESS;
-        *list = g_malloc0(size);
-    } else {
+    if (!ov->repeated_opts) {
         *list = NULL;
+        return false;
     }
+    ov->list_mode = LM_IN_PROGRESS;
+    *list = g_malloc0(size);
     return true;
 }
 
diff --git a/qapi/qapi-clone-visitor.c b/qapi/qapi-clone-visitor.c
index 5a54bd593f..c45c5caa3b 100644
--- a/qapi/qapi-clone-visitor.c
+++ b/qapi/qapi-clone-visitor.c
@@ -56,8 +56,7 @@ static bool qapi_clone_start_list(Visitor *v, const char *name,
                                   GenericList **listp, size_t size,
                                   Error **errp)
 {
-    qapi_clone_start_struct(v, name, (void **)listp, size, errp);
-    return true;
+    return qapi_clone_start_struct(v, name, (void **)listp, size, errp);
 }
 
 static GenericList *qapi_clone_next_list(Visitor *v, GenericList *tail,
@@ -75,8 +74,7 @@ static bool qapi_clone_start_alternate(Visitor *v, const char *name,
                                        GenericAlternate **obj, size_t size,
                                        Error **errp)
 {
-    qapi_clone_start_struct(v, name, (void **)obj, size, errp);
-    return true;
+    return qapi_clone_start_struct(v, name, (void **)obj, size, errp);
 }
 
 static bool qapi_clone_type_int64(Visitor *v, const char *name, int64_t *obj,
diff --git a/qdev-monitor.c b/qdev-monitor.c
index 7ebb97cf0d..31cc1f6564 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -652,7 +652,6 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
 
     /* Check whether the hotplug is allowed by the machine */
     if (qdev_hotplug && !qdev_hotplug_allowed(dev, errp)) {
-        /* Error must be set in the machine hook */
         goto err_del_dev;
     }
 
diff --git a/qom/object.c b/qom/object.c
index 8d698abf4d..d6bba48e41 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -1773,21 +1773,24 @@ static void object_set_link_property(Object *obj, Visitor *v,
     LinkProperty *prop = opaque;
     Object **targetp = object_link_get_targetp(obj, prop);
     Object *old_target = *targetp;
-    Object *new_target = NULL;
+    Object *new_target;
     char *path = NULL;
 
     if (!visit_type_str(v, name, &path, errp)) {
         return;
     }
 
-    if (strcmp(path, "") != 0) {
+    if (*path) {
         new_target = object_resolve_link(obj, name, path, errp);
+        if (!new_target) {
+            g_free(path);
+            return;
+        }
+    } else {
+        new_target = NULL;
     }
 
     g_free(path);
-    if (!new_target) {
-        return;
-    }
 
     prop->check(obj, name, new_target, &local_err);
     if (local_err) {

On 7/6/20 3:12 AM, Markus Armbruster wrote:
> diff between v2 rebased and v3, with hunks that change only whitespace
> dropped:

Thanks, that's useful.

Overall, the series looks ready to go from my perspective.  It looks 
like the changes from v2 were minimal enough that I don't have any R-b 
to add.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org