Mon 19 Jun 2017 03:56:41 PM UTC, original submission:
When parsing a "make -p" output, an unexpected "+=" was seen in a context where ":=" was expected (a target-specific variable being assigned with "=" to a pattern target, e.g. "%.x: FOO = x").
The bug is extremely unstable; it required the exact build directory, and a particular amount of data in the environment. Adding or removing unrelated environment variables ("XXX_<blah>") would cause the bug to come and go.
Running "valgrind --track-origins=yes" made the bug jump out:
Conditional jump or move depends on uninitialised value(s)
at 0x42265D: print_variable (variable.c:1664)
by 0x4243F0: print_variable_data_base (variable.c:1746)
by 0x415CC5: print_data_base (main.c:3367)
by 0x416809: die (main.c:3443)
by 0x407664: main (main.c:2581)
Uninitialised value was created by a heap allocation
at 0x4C27BE3: malloc (vg_replace_malloc.c:299)
by 0x417698: xmalloc (misc.c:221)
by 0x4228E1: create_pattern_var (variable.c:54)
by 0x41D047: record_target_var (read.c:1860)
by 0x41D047: eval (read.c:1178)
by 0x41D8F3: eval_makefile (read.c:437)
by 0x41CB58: eval (read.c:904)
by 0x41DE76: eval_buffer (read.c:480)
by 0x40E925: func_eval (function.c:1403)
by 0x410E18: handle_function (function.c:2527)
by 0x40B17E: variable_expand_string (expand.c:258)
by 0x40BC12: allocated_variable_expand_for_file (expand.c:564)
by 0x40EF0C: func_foreach (function.c:888)
Indeed, it does not appear that the variable structure allocated in create_pattern_var() is fully initialized. As a big hammer fix, I added a memset() for the embedded "struct variable" to set it to zero; with this change, the valgrind warning is fixed, and the flaky bug disappears (although it's unstable enough that that doesn't by itself prove anything). However, the valgrind warning points right to the code that chooses between ":=" and "+=" when printing the variable, so it seems like a strong hypothesis.
diff --git a/variable.c b/variable.c
index 364774f05064..3f962b160019 100644
--- a/variable.c
+++ b/variable.c
@@ -88,6 +88,7 @@ create_pattern_var (const char target, const char suffix)
p->target = target;
p->len = len;
p->suffix = suffix + 1;
+ memset(&p->variable, 0, sizeof(p->variable));
if (len < 256)
last_pattern_vars[len] = p;
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