Make breakpad build with -Wimplicit-fallthrough (on linux)

Fixes a bug where MD_EXCEPTION_CODE_MAC_PPC_ALTIVEC_ASSIST
would unintentionally get two reason strings appended.

Bug: 177475
Change-Id: I4957268328a242c7c75bbff8add98e9a48ba83ad
Reviewed-on: https://chromium-review.googlesource.com/895705
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/processor/minidump.cc

@@ -61,6 +61,22 @@
 #include "processor/basic_code_modules.h"
 #include "processor/logging.h"
 
+// All intentional fallthroughs in breakpad are in this file, so define
+// this macro locally.
+// If you ever move this to a .h file, make sure it's defined in a
+// private header file: clang suggests the first macro expanding to
+// [[clang::fallthrough]] in its diagnostics, so if BP_FALLTHROUGH
+// is visible in code depending on breakpad, clang would suggest
+// BP_FALLTHROUGH for code depending on breakpad, instead of the
+// client code's own fallthrough macro.
+// TODO(thakis): Once everyone uses C++17, use its [[fallthrough]] instead.
+#if defined(__clang__)
+#define BP_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define BP_FALLTHROUGH
+#endif
+
+
 namespace google_breakpad {
 
 
@@ -1969,6 +1985,7 @@ string MinidumpModule::code_identifier() const {
         break;
       }
       // Otherwise fall through to the case below.
+      BP_FALLTHROUGH;
     }
 
     case MD_OS_MAC_OS_X:
@@ -5199,7 +5216,7 @@ bool Minidump::Read() {
                             stream_type << ", but can only deal with one";
             return false;
           }
-          // Fall through to default
+          BP_FALLTHROUGH;
         }
 
         default: {

src/processor/minidump_processor.cc

@@ -893,6 +893,7 @@ string MinidumpProcessor::GetCrashReason(Minidump *dump, uint64_t *address) {
                   break;
                 case MD_EXCEPTION_CODE_MAC_PPC_ALTIVEC_ASSIST:
                   reason.append("EXC_PPC_ALTIVECASSIST");
+                  break;
                 default:
                   reason.append(flags_string);
                   BPLOG(INFO) << "Unknown exception reason " << reason;