profile: Protect .gcda output with flock

This avoids crashing or corrupting data if multiple concurrent
processes write to the same .gcda file. This is hard to test, since
the previous behaviour was a data race that often worked out, and it
ignores errors in flock to fall back to the old racy behaviour so that
it won't degrade the behaviour on filesystems that don't support
flock.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@234036 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/profile/GCDAProfiling.c b/lib/profile/GCDAProfiling.c
index 45fbd07e5..c1a9c3808 100644
--- a/lib/profile/GCDAProfiling.c
+++ b/lib/profile/GCDAProfiling.c
@@ -294,6 +294,11 @@ void llvm_gcda_start_file(const char *orig_filename, const char version[4],
     }
   }
 
+  /* Try to flock the file to serialize concurrent processes writing out to the
+   * same GCDA. This can fail if the filesystem doesn't support it, but in that
+   * case we'll just carry on with the old racy behaviour and hope for the best.
+   */
+  flock(fd, LOCK_EX);
   output_file = fdopen(fd, mode);
 
   /* Initialize the write buffer. */
@@ -493,6 +498,7 @@ void llvm_gcda_end_file() {
     }
 
     fclose(output_file);
+    flock(fd, LOCK_UN);
     output_file = NULL;
     write_buffer = NULL;
   }