Subject: [PATCH] die routine: change recursion limit from 1 to 1024

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 19 Jun 2017 22:00:36 +0000

Change the recursion limit for the default die routine from a *very*
low 1 to 1024. This ensures that infinite recursions are broken, but
doesn't lose error messages.

The intent of the existing code, as explained in commit
cd163d4b4e ("usage.c: detect recursion in die routines and bail out
immediately", 2012-11-14), is to break infinite recursion in cases
where the die routine itself dies.

However, doing that very aggressively by immediately printing out
"recursion detected in die handler" if we've already called die() once
means that threaded invocations of git can go through the following
flow:

 1. Start a bunch of threads

 2. The threads start invoking die(), pretty much at the same time.

 3. The first die() invocation will be in the middle of trying to
    print out its message by the time another thread dies, that other
    thread then runs into the recursion limit and dies with "recursion
    detected in die handler".

 4. Due to a race condition the initial error may never get printed
    before the "recursion detected" thread calls exit() and aborts the
    program.

An example of this is running a threaded grep against e.g. linux.git:

    git grep -P --threads=4 '(*LIMIT_RECURSION=1)(*LIMIT_MATCH=1)-?-?-?---$'

With the current version of git this will print some combination of
multiple PCRE failures that caused the abort and multiple "recursion
detected", some invocations will print out multiple "recursion
detected" errors with no PCRE error at all!

Now, git-grep could make use of the pluggable error facility added in
commit c19a490e37 ("usage: allow pluggable die-recursion checks",
2013-04-16).

That should be done for git-grep in particular because before this
change (and after) it'll potentially print out the exact same error
from the N threads it starts, that should be de-duplicated.

But let's start by improving the default behavior shared across all of
git. Right now the common case is not an infinite recursion in the
handler, but us losing error messages by default because we're overly
paranoid about our recursion check.

So let's just set the recursion limit to a number higher than the
number of threads we're ever likely to spawn. Now we won't lose
errors, and if we have a recursing die handler we'll still die within
microseconds.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>
---
 usage.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/usage.c b/usage.c
index 2f87ca69a8..1c198d4882 100644
--- a/usage.c
+++ b/usage.c
@@ -44,7 +44,9 @@ static void warn_builtin(const char *warn, va_list params)
 static int die_is_recursing_builtin(void)
 {
 	static int dying;
-	return dying++;
+	static int recursion_limit = 1024;
+
+	return dying++ > recursion_limit;
 }
 
 /* If we are in a dlopen()ed .so write to a global variable would segfault
-- 
2.13.1.518.g3df882009