linux: fix uv_exepath(size=1) UV_EINVAL error

uv_exepath() should write as much as possible to the output buffer.
In the case of size=1, skip the call to readlink() because it will
fail with EINVAL; just write the terminating zero byte.

PR-URL: https://github.com/libuv/libuv/pull/104
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/linux-core.c

@@ -381,7 +381,10 @@ int uv_exepath(char* buffer, size_t* size) {
   if (buffer == NULL || size == NULL || *size == 0)
     return -EINVAL;
 
-  n = readlink("/proc/self/exe", buffer, *size - 1);
+  n = *size - 1;
+  if (n > 0)
+    n = readlink("/proc/self/exe", buffer, n);
+
   if (n == -1)
     return -errno;
 

test/test-get-currentexe.c

@@ -73,5 +73,14 @@ TEST_IMPL(get_currentexe) {
   ASSERT(size == 0);
   ASSERT(buffer[0] == '\0');
 
+  memset(buffer, -1, sizeof(buffer));
+
+  size = 2;
+  r = uv_exepath(buffer, &size);
+  ASSERT(r == 0);
+  ASSERT(size == 1);
+  ASSERT(buffer[0] != '\0');
+  ASSERT(buffer[1] == '\0');
+
   return 0;
 }