[RFC PATCH 00/21] C99: show meaningful <file>:<line> in trace2 via macros

Since everyone's getting in on the C99 fun.

Well, $subject and a bit more. This RFC series has bits and pieces from thing I've submitted before. I'd proposed to make variadic macros a hard dependency before in [1] because I wanted to get to the goal in $subject, perhaps the whole thing will be more convincing.

This also includes the die_message() in a recent series of mine[2] that I abandoned.

At the end of this series we expose a config variable to have usage/die/warning emit line numbers. I.e. going from:

    $ git -c core.usageAddSource=false -c core.x=y config --get --bool core.x
    fatal: bad boolean config value 'y' for 'core.x'

To:

    $ git -c core.usageAddSource=false -c core.x=y config --get --bool core.x
    fatal: config.c:1241: bad boolean config value 'y' for 'core.x'

I find that to make tracing down errors in the test suite, and 21/21 has a GIT_TEST_* mode to turn it on there (which fails a lot now, but I'm hoping I'll eventually get passing).

But most importantly we've now got meaningful file/line numbers in
trace2 error events. I.e. from all of them being some line in usage.c:
    
    $ GIT_TRACE2_EVENT=/dev/stdout ~/g/git/git -c core.usageAddSource=false -c core.x=y config --get --bool core.x 2>&1 2>/dev/null|grep error | jq -r .
    {
      "event": "error",
      "sid": "20211115T221343.534151Z-Hc2f5b994-P003f3980",
      "thread": "main",
      "time": "2021-11-15T22:13:43.537981Z",
      "file": "usage.c",
      "line": 65,
      "msg": "bad boolean config value 'y' for 'core.x'",
      "fmt": "bad boolean config value '%s' for '%s'"
    }

To:
    
    $ GIT_TRACE2_EVENT=/dev/stdout ~/g/git/git -c core.usageAddSource=false -c core.x=y config --get --bool core.x 2>&1 2>/dev/null|grep error | jq -r .
    {
      "event": "error",
      "sid": "20211115T221357.083824Z-Hc2f5b994-P003f4a82",
      "thread": "main",
      "time": "2021-11-15T22:13:57.087596Z",
      "file": "config.c",
      "line": 1241,
      "msg": "bad boolean config value 'y' for 'core.x'",
      "fmt": "bad boolean config value '%s' for '%s'"
    }

I've got some speculation in 19/21 that this may make the "fmt" part redundant, i.e. did we only add that because we couldn't group these by file/line, but as noted there there's still some use-cases for "fmt" even with this series. In any case, this series doesn't touch that "fmt" key at all.

This is "RFC" mainly because there's a CI failure in 0061.2 with this, I still can't figure out what that's about (or if it's some fluke unrelated to this topic), but that has to be investigated.

But I wanted to see if people found the general idea interesting too. I picked the CC list mainly from paging through "--grep=trace2", and people who'd modified the tricker bits of usage.c code being modified here.

1. https://lore.kernel.org/git/cover-0.2-00000000000-20210412T105422Z-avarab@gmail.com/
2. https://lore.kernel.org/git/cover-v3-0.6-00000000000-20211022T175227Z-avarab@gmail.com/
3. https://github.com/avar/git/runs/4216916706?check_suite_focus=true

Ævar Arnfjörð Bjarmason (21):
  git-compat-util.h: clarify GCC v.s. C99-specific in comment
  C99 support: hard-depend on C99 variadic macros
  usage.c: add a die_message() routine
  usage.c API users: use die_message() where appropriate
  usage.c + gc: add and use a die_message_errno()
  config API: don't use vreportf(), make it static in usage.c
  common-main.c: call exit(), don't return
  usage.c: add a non-fatal bug() function to go with BUG()
  parse-options.[ch] API: use bug() to improve error output
  receive-pack: use bug() and BUG_if_bug()
  cache-tree.c: use bug() and BUG_if_bug()
  pack-objects: use BUG(...) not die("BUG: ...")
  strbuf.h: use BUG(...) not die("BUG: ...")
  usage API: create a new usage.h, move API docs there
  usage.[ch] API users: use report_fn, not hardcoded prototype
  usage.[ch] API: rename "warn" vars functions to "warning"
  usage.c: move usage routines around
  usage.c: move rename variables in usage routines around
  usage API: use C99 macros for {usage,usagef,die,error,warning,die}*()
  usage API: make the "{usage,fatal,error,warning,BUG}: " translatable
  usage API: add "core.usageAddSource" config to add <file>:<line>

 Documentation/CodingGuidelines                |   3 +
 Documentation/config/core.txt                 |   7 +
 .../technical/api-error-handling.txt          |  81 ------
 Documentation/technical/api-trace2.txt        |   4 +-
 apply.c                                       |   8 +-
 apply.h                                       |   6 +-
 banned.h                                      |   5 -
 builtin/fast-import.c                         |  22 +-
 builtin/gc.c                                  |  21 +-
 builtin/notes.c                               |  15 +-
 builtin/pack-objects.c                        |   2 +-
 builtin/receive-pack.c                        |  16 +-
 cache-tree.c                                  |   7 +-
 common-main.c                                 |   9 +-
 config.c                                      |  22 +-
 config.h                                      |  10 +-
 daemon.c                                      |   3 +-
 git-compat-util.h                             |  59 +---
 http-backend.c                                |   6 +-
 imap-send.c                                   |   3 +-
 parse-options.c                               |  56 ++--
 repo-settings.c                               |  11 +
 repository.h                                  |   2 +
 run-command.c                                 |  32 +--
 strbuf.h                                      |   2 +-
 t/helper/test-trace2.c                        |  27 +-
 t/t0210-trace2-normal.sh                      |  52 ++++
 trace.c                                       |  80 +-----
 trace.h                                       | 133 ++++-----
 trace2.c                                      |  45 +--
 trace2.h                                      |  28 --
 usage.c                                       | 270 +++++++++++-------
 usage.h                                       | 180 ++++++++++++
 33 files changed, 636 insertions(+), 591 deletions(-)
 delete mode 100644 Documentation/technical/api-error-handling.txt
 create mode 100644 usage.h