Dump gcov coverage in process_spawn (#10230)

Fixes https://github.com/neovim/neovim/pull/3926#issuecomment-502343527.

src/nvim/CMakeLists.txt

@@ -7,6 +7,7 @@ if(USE_GCOV)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --coverage")
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} --coverage")
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} --coverage")
+  add_compile_definitions(USE_GCOV)
 endif()
 endif()
 

src/nvim/event/process.c

@@ -22,6 +22,11 @@
 # include "event/process.c.generated.h"
 #endif
 
+/// Externally defined with gcov.
+#ifdef USE_GCOV
+void __gcov_dump(void);
+#endif
+
 // Time for a process to exit cleanly before we send KILL.
 // For PTY processes SIGTERM is sent first (in case SIGHUP was not enough).
 #define KILL_TIMEOUT_MS 2000
@@ -50,6 +55,11 @@ int process_spawn(Process *proc, bool in, bool out, bool err)
     proc->err.closed = true;
   }
 
+#ifdef USE_GCOV
+  // Dump coverage data before forking, to avoid "Merge mismatch" errors.
+  __gcov_dump();
+#endif
+
   int status;
   switch (proc->type) {
     case kProcessTypeUv: