loader: Change strcpy_sx() to be limited version of strcpy_s()

The strcpy_sx() now reports errors and behaves as strcpy_s() when buffer is too small (nulls the buffer). But it does not have all the checks.

lib/loader/loader.c

@@ -54,14 +54,21 @@
 #if !defined(EVMC_LOADER_MOCK)
 static
 #endif
-    void
+    int
     strcpy_sx(char* restrict dest, size_t destsz, const char* restrict src)
 {
     size_t len = strlen(src);
-    if (len > destsz - 1)
-        len = destsz - 1;
+    if (len >= destsz)
+    {
+        // The input src will not fit into the dest buffer.
+        // Set the first byte of the dest to null to make it effectively empty string
+        // and return error.
+        dest[0] = 0;
+        return 1;
+    }
     memcpy(dest, src, len);
     dest[len] = 0;
+    return 0;
 }
 #endif
 

test/unittests/test_loader.cpp

@@ -20,7 +20,7 @@ extern "C" {
 #define strcpy_sx strcpy_s
 #else
 /// Declaration of internal function defined in loader.c.
-void strcpy_sx(char* dest, size_t destsz, const char* src);
+int strcpy_sx(char* dest, size_t destsz, const char* src);
 #endif
 
 /// The library path expected by mocked evmc_test_load_library().
@@ -127,13 +127,24 @@ static evmc_instance* create_failure()
 
 TEST_F(loader, strcpy_sx)
 {
-    const auto input = "12";
-    char buf[2] = {0x0f, 0x0e};
-    static_assert(sizeof(input) > sizeof(buf), "");
+    const char input_empty[] = "";
+    const char input_that_fits[] = "x";
+    const char input_too_big[] = "12";
+    char buf[2] = {0x0f, 0x0f};
+    static_assert(sizeof(input_empty) <= sizeof(buf), "");
+    static_assert(sizeof(input_that_fits) <= sizeof(buf), "");
+    static_assert(sizeof(input_too_big) > sizeof(buf), "");
 
-    strcpy_sx(buf, sizeof(buf), input);
+    EXPECT_EQ(strcpy_sx(buf, sizeof(buf), input_empty), 0);
     EXPECT_EQ(buf[0], 0);
+    EXPECT_EQ(buf[1], 0x0f);
+
+    EXPECT_EQ(strcpy_sx(buf, sizeof(buf), input_that_fits), 0);
+    EXPECT_EQ(buf[0], 'x');
     EXPECT_EQ(buf[1], 0);
+
+    EXPECT_NE(strcpy_sx(buf, sizeof(buf), input_too_big), 0);
+    EXPECT_EQ(buf[0], 0);
 }
 
 TEST_F(loader, load_nonexistent)