Parse lock file leniently

CHANGELOG.md

@@ -8,6 +8,8 @@
 - Add `--stdin-filepath` flag to `buf format`, which reads a single `.proto` file from
   stdin and writes the formatted result to stdout. The path is not read from disk, and is
   only used to report parse errors and diffs.
+- Parse v2 `buf.lock` files non-strictly, ignoring unknown fields. A `buf.lock` written by a
+  newer `buf` that records additional fields no longer fails to parse on an older `buf`.
 
 ## [v1.73.0] - 2026-09-11
 

private/bufpkg/bufconfig/buf_lock_file.go

@@ -405,7 +405,9 @@ func readBufLockFile(
 		return newBufLockFile(fileVersion, objectData, depModuleKeys, nil /* remotePluginKeys */, nil /* remotePolicyKeys */, nil /* remotePolicyPluginKeys */)
 	case FileVersionV2:
 		var externalBufLockFile externalBufLockFileV2
-		if err := getUnmarshalStrict(allowJSON)(data, &externalBufLockFile); err != nil {
+		// Parse non-strict to allow unknown fields possibly written by a newer version of buf.
+		// Unknown fields are dropped and not written back.
+		if err := getUnmarshalNonStrict(allowJSON)(data, &externalBufLockFile); err != nil {
 			return nil, err
 		}
 		depModuleKeys := make([]bufmodule.ModuleKey, len(externalBufLockFile.Deps))

private/bufpkg/bufconfig/buf_lock_file_test.go

@@ -122,6 +122,77 @@ policies:
 	)
 }
 
+// TestReadBufLockFileIgnoresUnknownFields covers that a buf.lock written by a newer buf
+// with fields this version does not know is still readable, and that the unknown fields
+// are dropped rather than written back.
+func TestReadBufLockFileIgnoresUnknownFields(t *testing.T) {
+	t.Parallel()
+
+	testReadWriteBufLockFileRoundTrip(
+		t,
+		// input
+		`version: v2
+future_top_level_field: value
+deps:
+  - name: buf.testing/acme/date
+    future_dep_field: value
+    commit: ffded0b4cf6b47cab74da08d291a3c2f
+    digest: b5:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+plugins:
+  - name: buf.testing/acme/plugin
+    future_plugin_field: value
+    commit: ffded0b4cf6b47cab74da08d291a3c2f
+    digest: p1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+policies:
+  - name: buf.testing/acme/policy
+    future_policy_field: value
+    commit: b8488077ea6d4f6d9562a337b98259c8
+    digest: o1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+    plugins:
+      - name: buf.testing/acme/plugin
+        future_policy_plugin_field: value
+        commit: ffded0b4cf6b47cab74da08d291a3c2f
+        digest: p1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+`,
+		// expected output
+		`version: v2
+deps:
+  - name: buf.testing/acme/date
+    commit: ffded0b4cf6b47cab74da08d291a3c2f
+    digest: b5:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+plugins:
+  - name: buf.testing/acme/plugin
+    commit: ffded0b4cf6b47cab74da08d291a3c2f
+    digest: p1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+policies:
+  - name: buf.testing/acme/policy
+    commit: b8488077ea6d4f6d9562a337b98259c8
+    digest: o1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+    plugins:
+      - name: buf.testing/acme/plugin
+        commit: ffded0b4cf6b47cab74da08d291a3c2f
+        digest: p1:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+`,
+	)
+
+	// v1 buf.lock files are still parsed strictly. They are no longer written, so there
+	// is no newer buf that could add a field to them.
+	_, err := ReadBufLockFile(
+		t.Context(),
+		strings.NewReader(testCleanYAMLData(`version: v1
+future_top_level_field: value
+deps:
+  - remote: buf.testing
+    owner: acme
+    repository: date
+    commit: ffded0b4cf6b47cab74da08d291a3c2f
+    digest: shake256:24ed4f13925cf89ea0ae0127fa28540704c7ae14750af027270221b737a1ce658f8014ca2555f6f7fcd95ea84e071d33f37f86cc36d07fe0d0963329a5ec2462
+`)),
+		DefaultBufLockFileName,
+	)
+	require.ErrorContains(t, err, "future_top_level_field")
+}
+
 func testReadBufLockFile(
 	t *testing.T,
 	inputBufLockFileData string,