fix(blueprint): Detect binary files during three way merges (#410)

The three way merge is text based and won't produce meaningful results
when run on binary files. Use a heuristic similar to git of checking the
first 3000 bytes of the input files for null bytes. If there are any null
bytes, consider the file to be binary and fall back to returning the proposed
file.

packages/blueprints/blueprint/src/resynthesis/merge-strategies/merge-strategies.ts

@@ -52,20 +52,29 @@ export class MergeStrategies {
    * A strategy that performs a three way merge between the existing, proposed, and common
    * ancestor files. The resolved file may contain conflict markers if the files can not be
    * cleanly merged.
+   *
+   * The provided files' contents must be UTF-8 encoded. The strategy attempts to detect
+   * if the input files are binary and will return the proposedFile if any of the files
+   * are binary.
    */
   public static threeWayMerge: StrategyFunction = function threeWayMerge(
     commonAncestorFile: ContextFile | undefined,
     existingFile: ContextFile | undefined,
     proposedFile: ContextFile | undefined,
   ) {
+    // The three way merge algorithm below is text based and won't produce a meaningful result
+    // when merging binary files. Default to the proposed file when we detect one of the files is
+    // binary:
+    if (isBinaryFile(proposedFile) || isBinaryFile(existingFile) || isBinaryFile(commonAncestorFile)) {
+      return proposedFile;
+    }
+
     if (!existingFile && proposedFile && commonAncestorFile && proposedFile.buffer.equals(commonAncestorFile.buffer)) {
       return undefined;
     }
-
     if (!proposedFile && existingFile && commonAncestorFile && existingFile.buffer.equals(commonAncestorFile.buffer)) {
       return undefined;
     }
-
     if (!existingFile && !proposedFile) {
       return undefined;
     }
@@ -97,3 +106,24 @@ export class MergeStrategies {
     };
   };
 }
+
+const BINARY_FILE_HEURISTIC_MAX_LENGTH = 8000;
+
+/**
+ * Returns whether the given ContextFile is binary or not using a heuristic based
+ * approach of checking for null bytes in the beginning of the file contents.
+ */
+function isBinaryFile(file?: ContextFile): boolean {
+  if (!file) {
+    return false;
+  }
+
+  const searchLength = Math.min(file.buffer.length, BINARY_FILE_HEURISTIC_MAX_LENGTH);
+  for (let i = 0; i < searchLength; i++) {
+    if (file.buffer[i] === 0) {
+      return true;
+    }
+  }
+
+  return false;
+}