Treat bad values passed to --images as a fatal error

tools/flags/SkCommonFlags.cpp

Index: tools/flags/SkCommonFlags.cpp
diff --git a/tools/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index a2da1314fc637575764703f2ff0cdd8de524e2ee..e29c824a7ac1f470438b7f12fb36dc74914c8558 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "SkCommonFlags.h"
+#include "SkOSFile.h"
 
 DEFINE_bool(cpu, true, "master switch for running CPU-bound work.");
 
@@ -14,7 +15,8 @@ DEFINE_bool(dryRun, false,
 
 DEFINE_bool(gpu, true, "master switch for running GPU-bound work.");
 
-DEFINE_string(images, "", "Directory of images to decode.");
+DEFINE_string(images, "", "List of images and/or directories to decode. A directory with no images"
+                          " is treated as a fatal error.");
 
 DEFINE_string2(match, m, nullptr,
                "[~][^]substring[$] [...] of GM name to run.\n"
@@ -48,3 +50,40 @@ DEFINE_string(key, "",
 DEFINE_string(properties, "",
               "Space-separated key/value pairs to add to JSON identifying this run.");
 
+namespace SkCommonFlags {
+    void collectImages(SkTArray<SkString>* output) {
+        SkASSERT(output);
+
+        static const char* const exts[] = {
+            "bmp", "gif", "jpg", "jpeg", "png", "webp", "ktx", "astc", "wbmp", "ico",
+            "BMP", "GIF", "JPG", "JPEG", "PNG", "WEBP", "KTX", "ASTC", "WBMP", "ICO",
+        };
+
+        for (int i = 0; i < FLAGS_images.count(); ++i) {
+            const char* flag = FLAGS_images[i];
+            if (!sk_exists(flag)) {
+                SkDebugf("%s does not exits\n", flag);
+            }
+
+            if (sk_isdir(flag)) {
+                // If the value passed in is a directory, add all the images
+                bool foundAnImage = false;
+                for (const char* ext : exts) {
+                    SkOSFile::Iter it(flag, ext);
+                    SkString file;
+                    while (it.next(&file)) {
+                        foundAnImage = true;
+                        output->push_back() = SkOSPath::Join(flag, file.c_str());
+                    }
+                }
+                if (!foundAnImage) {
+                    SkDebugf("No supported images found in %s! Exiting\n", flag);
+                    exit(1);
+                }
+            } else {
+                // Also add the value if it is a single image
+                output->push_back() = flag;
+            }
+        }
+    }
+}