Bug 1705641 - Don't flush ccov counters when process is dying when env var MOZ_FUZZING_CCOV is defined r=andi

Sometimes there is a deadlock between user-requested ccov counters flush (through a signal) and flush happening when a process is dying.
So in order to avoid it, just remove the latter which is out of our control (user-requested flushes are protected using a cross mutex).

Differential Revision: https://phabricator.services.mozilla.com/D112632

build/build-clang/clang-12-linux64.json

@@ -19,6 +19,7 @@
       "downgrade-mangling-error_clang_12.patch",
       "revert-llvmorg-12-init-7827-g2a078c307204.patch",
       "tsan-D101154.patch",
-      "loosen-msvc-detection.patch"
+      "loosen-msvc-detection.patch",
+      "fuzzing_ccov_build_clang_12.patch"
     ]
 }

build/build-clang/fuzzing_ccov_build_clang_12.patch

@@ -0,0 +1,27 @@
+From 98bf90ef5ea3dd848ce7d81a662eb7499d11c91c Mon Sep 17 00:00:00 2001
+From: Calixte Denizet <calixte.denizet@gmail.com>
+Date: Fri, 16 Apr 2021 10:05:34 +0200
+Subject: [PATCH] [Gcov] Don't run global destructor in ccov builds when env
+ MOZ_FUZZING_CCOV is existing
+
+---
+ compiler-rt/lib/profile/GCDAProfiling.c | 3 +++
+ 1 file changed, 3 insertions(+)
+
+diff --git a/compiler-rt/lib/profile/GCDAProfiling.c b/compiler-rt/lib/profile/GCDAProfiling.c
+index 4293e8f7b5bf..6cda4bc7601f 100644
+--- a/compiler-rt/lib/profile/GCDAProfiling.c
++++ b/compiler-rt/lib/profile/GCDAProfiling.c
+@@ -586,6 +586,9 @@ void llvm_writeout_files(void) {
+ __attribute__((destructor(100)))
+ #endif
+ static void llvm_writeout_and_clear(void) {
++  if (getenv("MOZ_FUZZING_CCOV")) {
++    return;
++  }
+   llvm_writeout_files();
+   fn_list_remove(&writeout_fn_list);
+ }
+-- 
+2.30.2
+