Bug 1517433 - do not assert on over-long float printf; r=froydnj

mozilla::PrintfTarget::cvt_f release asserts that the desired printf
fit into a statically-sized buffer.  However, this may not be the case
if the user requested a larger width or precision.  Handle this
unusual case by allocating a temporary buffer.

MozReview-Commit-ID: 2WicecHDzDR

Differential Revision: https://phabricator.services.mozilla.com/D15989

--HG--
extra : moz-landing-system : lando

mozglue/misc/Printf.cpp

@@ -11,6 +11,7 @@
  */
 
 #include "mozilla/AllocPolicy.h"
+#include "mozilla/Likely.h"
 #include "mozilla/Printf.h"
 #include "mozilla/Sprintf.h"
 #include "mozilla/UniquePtrExtensions.h"
@@ -258,9 +259,27 @@ bool mozilla::PrintfTarget::cvt_f(double d, const char* fmt0,
   }
 #endif
   size_t len = SprintfLiteral(fout, fin, d);
-  MOZ_RELEASE_ASSERT(len <= sizeof(fout));
+  // Note that SprintfLiteral will always write a \0 at the end, so a
+  // "<=" check here would be incorrect -- the buffer size passed to
+  // snprintf includes the trailing \0, but the returned length does
+  // not.
+  if (MOZ_LIKELY(len < sizeof(fout))) {
+    return emit(fout, len);
+  }
 
-  return emit(fout, len);
+  // Maybe the user used "%500.500f" or something like that.
+  size_t buf_size = len + 1;
+  UniqueFreePtr<char> buf((char*)malloc(buf_size));
+  if (!buf) {
+    return false;
+  }
+  len = snprintf(buf.get(), buf_size, fin, d);
+  // If this assert fails, then SprintfLiteral has a bug -- and in
+  // this case we would like to learn of it, which is why there is a
+  // release assert.
+  MOZ_RELEASE_ASSERT(len < buf_size);
+
+  return emit(buf.get(), len);
 }
 
 /*

mozglue/tests/TestPrintf.cpp

@@ -122,6 +122,11 @@ static void TestPrintfFormats() {
   // failure caused by printing a very long floating point value.
   print_one("ignore", "%lf", DBL_MAX);
 
+  // Regression test for bug#1517433.  The bug was an assertion
+  // failure caused by printing a floating point value with a large
+  // precision and/or width.
+  print_one("ignore", "%500.500lf", DBL_MAX);
+
   MOZ_RELEASE_ASSERT(print_one("2727", "%" PRIu32, (uint32_t)2727));
   MOZ_RELEASE_ASSERT(print_one("aa7", "%" PRIx32, (uint32_t)2727));
   MOZ_RELEASE_ASSERT(print_one("2727", "%" PRIu64, (uint64_t)2727));