threads / rfc / 54694

RFC patchusage: add trace2 entry upon warning()

Subject: [RFC PATCH] usage: add trace2 entry upon warning()

## tl;dr

6 messages between Nov 23, 2020 and Nov 30, 2020. Diffs are folded; open one to read it.

replies: 5people: 3as markdown or json

Jonathan Tan· Nov 23, 2020, 19:04 UTC · lore

Emit a trace2 error event whenever warning() is called, just like when die(), error(), or usage() is called.

This helps debugging issues that would trigger warnings but not errors. In particular, this might have helped debugging an issue I encountered with commit graphs at $DAYJOB [1].

There is a tradeoff between including potentially relevant messages and cluttering up the trace output produced. I think that warning() messages should be included in traces, because by its nature, Git is used over multiple invocations of the Git tool, and a failure (currently traced) in a Git invocation might be caused by an unexpected interaction in a previous Git invocation that only has a warning (currently untraced) as a symptom - as is the case in [1].

[1] https://lore.kernel.org/git/20200629220744.1054093-1-jonathantanmy@google.com/
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
The documentation (see the patch below) says that it is emitted when
usage() is called, but I don't see it in the code (nor in practice). If
not emitting traces upon usage() is intended (which is reasonable to
me), I'll edit the documentation (and the commit message above).
I marked this as RFC mostly because this is a perhaps subjective
tradeoff that needs to be made. I think the tradeoff should be in favor
of tracing warning() messages, and I have explained that in the commit
message.
---
 Documentation/technical/api-trace2.txt | 2 +-
 usage.c                                | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)
Show changes to 2 files +7 −1

Documentation/technical/api-trace2.txt, usage.c

diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
index 6b6085585d..c65ffafc48 100644
--- a/Documentation/technical/api-trace2.txt
+++ b/Documentation/technical/api-trace2.txt
@@ -466,7 +466,7 @@ completed.)
 
 `"error"`::
 	This event is emitted when one of the `error()`, `die()`,
-	or `usage()` functions are called.
+	`warning()`, or `usage()` functions are called.
 +
 ------------
 {
diff --git a/usage.c b/usage.c
index 06665823a2..673edf5afd 100644
--- a/usage.c
+++ b/usage.c
@@ -81,6 +81,12 @@ static void error_builtin(const char *err, va_list params)
 
 static void warn_builtin(const char *warn, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(err, params);
+
 	vreportf("warning: ", warn, params);
 }
 

-- 
2.29.2.454.gaff20da3a2-goog
Jonathan Tan· Nov 23, 2020, 20:45 UTC · re: Jonathan Tan · lore

[RFC PATCH v2] usage: add trace2 entry upon warning()

Emit a trace2 error event whenever warning() is called, just like when die(), error(), or usage() is called.

This helps debugging issues that would trigger warnings but not errors. In particular, this might have helped debugging an issue I encountered with commit graphs at $DAYJOB [1].

There is a tradeoff between including potentially relevant messages and cluttering up the trace output produced. I think that warning() messages should be included in traces, because by its nature, Git is used over multiple invocations of the Git tool, and a failure (currently traced) in a Git invocation might be caused by an unexpected interaction in a previous Git invocation that only has a warning (currently untraced) as a symptom - as is the case in [1].

[1] https://lore.kernel.org/git/20200629220744.1054093-1-jonathantanmy@google.com/
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
Whoops...I ran a compile for the first version but I sent the email out
before looking at the results. The first version had a compile error,
but this one should be fine.
---
 Documentation/technical/api-trace2.txt | 2 +-
 usage.c                                | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)
Show changes to 2 files +7 −1

Documentation/technical/api-trace2.txt, usage.c

diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
index 6b6085585d..c65ffafc48 100644
--- a/Documentation/technical/api-trace2.txt
+++ b/Documentation/technical/api-trace2.txt
@@ -466,7 +466,7 @@ completed.)
 
 `"error"`::
 	This event is emitted when one of the `error()`, `die()`,
-	or `usage()` functions are called.
+	`warning()`, or `usage()` functions are called.
 +
 ------------
 {
diff --git a/usage.c b/usage.c
index 06665823a2..1868a24f7a 100644
--- a/usage.c
+++ b/usage.c
@@ -81,6 +81,12 @@ static void error_builtin(const char *err, va_list params)
 
 static void warn_builtin(const char *warn, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(warn, params);
+
 	vreportf("warning: ", warn, params);
 }
 

-- 
2.29.2.454.gaff20da3a2-goog
Junio C Hamano· Nov 23, 2020, 22:30 UTC · re: Jonathan Tan · lore

Re: [RFC PATCH v2] usage: add trace2 entry upon warning()

Jonathan Tan <jonathantanmy@google.com> writes:
Show 15 quoted lines
> diff --git a/usage.c b/usage.c
> index 06665823a2..1868a24f7a 100644
> --- a/usage.c
> +++ b/usage.c
> @@ -81,6 +81,12 @@ static void error_builtin(const char *err, va_list params)
>  
>  static void warn_builtin(const char *warn, va_list params)
>  {
> +	/*
> +	 * We call this trace2 function first and expect it to va_copy 'params'
> +	 * before using it (because an 'ap' can only be walked once).
> +	 */
> +	trace2_cmd_error_va(warn, params);
> +
>  	vreportf("warning: ", warn, params);

Without the comment, this would have been a huge red sign. Given what the trace2 function does, I think cmd_error_va() has to do the va_copy()/va_end() dance somewhere in it, so from the caller's point of view, not having to va_copy()/va_end() is convenient and nice, but perhaps we can eventually (not today---we have the same issue with die_builtin() and error_builtin()) move the comment to the callee so that other people who are writing more callers do not have to rediscover what to do with "can va_lsit be used more than once?" issue. Perhaps like (I am not demonstrating the removal of dups):

 trace2.h | 5 +++++
 1 file changed, 5 insertions(+)
Show changes to diff +5 −0
diff --git c/trace2.h w/trace2.h
index b18bc5529c..10ba231122 100644
--- c/trace2.h
+++ w/trace2.h
@@ -116,6 +116,11 @@ int trace2_cmd_exit_fl(const char *file, int line, int code);
  * Emit an 'error' event.
  *
  * Write an error message to the TRACE2 targets.
+ *
+ * Note that the "va_list ap" the caller has is not touched, because
+ * it is fed to each of the TRACE2 targets, which must use va_copy()
+ * and use va_end() on the copy.  The caller can still use ap it uses
+ * to call this funciton and and call va_end() on it when it is done.
  */
 void trace2_cmd_error_va_fl(const char *file, int line, const char *fmt,
 			    va_list ap);
Jonathan Tan· Nov 24, 2020, 20:05 UTC · re: Jonathan Tan · lore

[PATCH v3] usage: add trace2 entry upon warning()

Emit a trace2 error event whenever warning() is called, just like when die(), error(), or usage() is called.

This helps debugging issues that would trigger warnings but not errors. In particular, this might have helped debugging an issue I encountered with commit graphs at $DAYJOB [1].

There is a tradeoff between including potentially relevant messages and cluttering up the trace output produced. I think that warning() messages should be included in traces, because by its nature, Git is used over multiple invocations of the Git tool, and a failure (currently traced) in a Git invocation might be caused by an unexpected interaction in a previous Git invocation that only has a warning (currently untraced) as a symptom - as is the case in [1].

[1] https://lore.kernel.org/git/20200629220744.1054093-1-jonathantanmy@google.com/
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
Thanks, Junio. That comment looks good. Here is the version with Junio's
suggested comment included, for everyone's reference.
---
 Documentation/technical/api-trace2.txt | 2 +-
 trace2.h                               | 5 +++++
 usage.c                                | 6 ++++++
 3 files changed, 12 insertions(+), 1 deletion(-)
Show changes to 3 files +12 −1

Documentation/technical/api-trace2.txt, trace2.h, usage.c

diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
index 6b6085585d..c65ffafc48 100644
--- a/Documentation/technical/api-trace2.txt
+++ b/Documentation/technical/api-trace2.txt
@@ -466,7 +466,7 @@ completed.)
 
 `"error"`::
 	This event is emitted when one of the `error()`, `die()`,
-	or `usage()` functions are called.
+	`warning()`, or `usage()` functions are called.
 +
 ------------
 {
diff --git a/trace2.h b/trace2.h
index b18bc5529c..2b5c5c4ba0 100644
--- a/trace2.h
+++ b/trace2.h
@@ -116,6 +116,11 @@ int trace2_cmd_exit_fl(const char *file, int line, int code);
  * Emit an 'error' event.
  *
  * Write an error message to the TRACE2 targets.
+ * 
+ * Note that the "va_list ap" the caller has is not touched, because
+ * it is fed to each of the TRACE2 targets, which must use va_copy()
+ * and use va_end() on the copy.  The caller can still use ap it uses
+ * to call this function and and call va_end() on it when it is done.
  */
 void trace2_cmd_error_va_fl(const char *file, int line, const char *fmt,
 			    va_list ap);
diff --git a/usage.c b/usage.c
index 06665823a2..1868a24f7a 100644
--- a/usage.c
+++ b/usage.c
@@ -81,6 +81,12 @@ static void error_builtin(const char *err, va_list params)
 
 static void warn_builtin(const char *warn, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(warn, params);
+
 	vreportf("warning: ", warn, params);
 }
 

-- 
2.29.2.454.gaff20da3a2-goog
Junio C Hamano· Nov 24, 2020, 22:15 UTC · re: Jonathan Tan · lore

Re: [PATCH v3] usage: add trace2 entry upon warning()

Jonathan Tan <jonathantanmy@google.com> writes:
Show 21 quoted lines
> Emit a trace2 error event whenever warning() is called, just like when
> die(), error(), or usage() is called.
>
> This helps debugging issues that would trigger warnings but not errors.
> In particular, this might have helped debugging an issue I encountered
> with commit graphs at $DAYJOB [1].
>
> There is a tradeoff between including potentially relevant messages and
> cluttering up the trace output produced. I think that warning() messages
> should be included in traces, because by its nature, Git is used over
> multiple invocations of the Git tool, and a failure (currently traced)
> in a Git invocation might be caused by an unexpected interaction in a
> previous Git invocation that only has a warning (currently untraced) as
> a symptom - as is the case in [1].
>
> [1] https://lore.kernel.org/git/20200629220744.1054093-1-jonathantanmy@google.com/
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
> Thanks, Junio. That comment looks good. Here is the version with Junio's
> suggested comment included, for everyone's reference.

Heh, I meant it as more of a #leftoverbit, not directly applicable to this particular patch, but would be a good follow-up topic, as I would have expected that die/warn/error should lose their own comments where they call trace2_cmd_error_va() in the same patch that adds the comment for callers near the function.

Let's use v2 if the difference between v2 and v3 is only the addition of the comment before trace2_cmd_error_va() function decl to help the callers.

Jeff Hostetler· Nov 30, 2020, 16:16 UTC · re: Jonathan Tan · lore

Re: [RFC PATCH] usage: add trace2 entry upon warning()

On 11/23/20 2:04 PM, Jonathan Tan wrote:
Show 18 quoted lines
> Emit a trace2 error event whenever warning() is called, just like when
> die(), error(), or usage() is called.
> 
> This helps debugging issues that would trigger warnings but not errors.
> In particular, this might have helped debugging an issue I encountered
> with commit graphs at $DAYJOB [1].
> 
> There is a tradeoff between including potentially relevant messages and
> cluttering up the trace output produced. I think that warning() messages
> should be included in traces, because by its nature, Git is used over
> multiple invocations of the Git tool, and a failure (currently traced)
> in a Git invocation might be caused by an unexpected interaction in a
> previous Git invocation that only has a warning (currently untraced) as
> a symptom - as is the case in [1].
> 
> [1] https://lore.kernel.org/git/20200629220744.1054093-1-jonathantanmy@google.com/
> 
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
LGTM

Thanks, Jeff

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