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

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 <avarab@gmail.com>
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 usage.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)