fix(splat): stop mutating the shared info[SPLAT] array

splat.js

@@ -54,9 +54,15 @@ class Splatter {
     const expectedSplat = tokens.length - escapes;
     const extraSplat = expectedSplat - splat.length;
     const metas = extraSplat < 0
-      ? splat.splice(extraSplat, -1 * extraSplat)
+      ? splat.slice(extraSplat)
       : [];
 
+    // `splat` may be the same array referenced by info[SPLAT], which is
+    // shared across every transport's format pipeline for this log call -
+    // mutating it (e.g. via splice) here would drop the extra metadata for
+    // every transport after the first, so this only ever reads from it.
+    const formatArgs = extraSplat < 0 ? splat.slice(0, extraSplat) : splat;
+
     // Now that { splat } has been separated from any potential { meta }. we
     // can assign this to the `info` object and write it to our format stream.
     // If the additional metas are **NOT** objects or **LACK** enumerable properties
@@ -68,7 +74,7 @@ class Splatter {
       }
     }
 
-    info.message = util.format(msg, ...splat);
+    info.message = util.format(msg, ...formatArgs);
     return info;
   }
 
@@ -97,8 +103,10 @@ class Splatter {
     // This condition will take care of inputs with info[SPLAT]
     // but no tokens present
     if (!tokens && (splat || splat.length)) {
+      // Read-only: splat may be the same array referenced by info[SPLAT],
+      // shared across every transport's format pipeline for this log call.
       const metas = splat.length > 1
-        ? splat.splice(0)
+        ? splat.slice(0)
         : splat;
 
       // Now that { splat } has been separated from any potential { meta }. we

test/splat.test.js

@@ -155,4 +155,37 @@ describe('splat', () => {
     }
   ));
 
+  // See: https://github.com/winstonjs/winston/issues/2365
+  // winston shallow-clones info per transport, so each transport's format
+  // pipeline gets its own info object but the same info[SPLAT] array
+  // reference. Mutating that array in place during the first transport's
+  // pass silently drops the meta for every subsequent transport.
+  it('does not mutate info[SPLAT], so a second transport still sees the meta', () => {
+    const sharedSplat = ['ball', 'cool', { extraData: 'traceId' }];
+    const infoA = { level: 'info', message: 'this %s is %s', [SPLAT]: sharedSplat };
+    const infoB = { level: 'info', message: 'this %s is %s', [SPLAT]: sharedSplat };
+
+    const first = splat().transform(infoA);
+    assume(first.message).equals('this ball is cool');
+    assume(first.extraData).equals('traceId');
+
+    const second = splat().transform(infoB);
+    assume(second.message).equals('this ball is cool');
+    assume(second.extraData).equals('traceId');
+  });
+
+  it('no tokens | does not mutate info[SPLAT] either', () => {
+    const sharedSplat = [{ extraData: 'traceId' }, { more: 'meta' }];
+    const infoA = { level: 'info', message: 'no tokens here', [SPLAT]: sharedSplat };
+    const infoB = { level: 'info', message: 'no tokens here', [SPLAT]: sharedSplat };
+
+    const first = splat().transform(infoA);
+    assume(first.extraData).equals('traceId');
+    assume(first.more).equals('meta');
+
+    const second = splat().transform(infoB);
+    assume(second.extraData).equals('traceId');
+    assume(second.more).equals('meta');
+  });
+
 });