fix: Race condition on downloading the same file twice

Author: wayfarer3130

test/data-options.test.js

@@ -169,7 +169,7 @@ it("noCopy test_fragment_multiframe", async () => {
         "https://github.com/dcmjs-org/data/releases/download/encapsulation/encapsulation-fragment-multiframe.dcm";
     const dcmPath = await getTestDataset(
         url,
-        "encapsulation-fragment-multiframe-b.dcm"
+        "encapsulation-fragment-multiframe.dcm"
     );
     const file = fs.readFileSync(dcmPath);
 

test/testUtils.js

@@ -55,22 +55,31 @@ function ensureTestDataDir() {
 }
 
 async function getZippedTestDataset(url, filename, unpackDirectory) {
-    var dir = ensureTestDataDir();
-    var targetPath = path.join(dir, filename);
-    var unpackPath = path.join(dir, unpackDirectory);
+    const dir = ensureTestDataDir();
+    const targetPath = path.join(dir, filename);
+    const unpackPath = path.join(dir, unpackDirectory);
     if (!fs.existsSync(unpackPath)) {
         await downloadToFile(url, targetPath);
         await unzip(targetPath, unpackPath);
     }
     return unpackPath;
 }
 
+/**
+ * Stores the required downloads to prevent async reading before download completed.
+ */
+const asyncDownloadMap = new Map();
+
 async function getTestDataset(url, filename) {
-    var dir = ensureTestDataDir();
-    var targetPath = path.join(dir, filename);
-    if (!fs.existsSync(targetPath)) {
-        await downloadToFile(url, targetPath);
+    const dir = ensureTestDataDir();
+    const targetPath = path.join(dir, filename);
+    let filePromise = asyncDownloadMap.get(targetPath);
+    if (!filePromise && !fs.existsSync(targetPath)) {
+        filePromise = downloadToFile(url, targetPath);
+        asyncDownloadMap.set(targetPath,filePromise);
     }
+    // This returns immediately if filePromise is undefined - eg if the file already downloaded.
+    await filePromise;
     return targetPath;
 }