fix(cli): resolve race condition in locale processing

.changeset/all-candies-reply.md

@@ -0,0 +1,5 @@
+---
+"lingo.dev": patch
+---
+
+fix racing condition where concurrent processing could use data from the wrong locale

packages/cli/src/cli/loaders/_utils.ts

@@ -48,8 +48,10 @@ export function createLoader<I, O, C>(
   const state = {
     defaultLocale: undefined as string | undefined,
     originalInput: undefined as I | undefined | null,
-    pullInput: undefined as I | undefined | null,
-    pullOutput: undefined as O | undefined | null,
+    // Store pullInput and pullOutput per-locale to avoid race conditions
+    // when multiple locales are processed concurrently
+    pullInputByLocale: new Map<string, I | null>(),
+    pullOutputByLocale: new Map<string, O | null>(),
     initCtx: undefined as C | undefined,
   };
   return {
@@ -81,15 +83,15 @@ export function createLoader<I, O, C>(
         state.originalInput = input || null;
       }
 
-      state.pullInput = input;
+      state.pullInputByLocale.set(locale, input || null);
       const result = await lDefinition.pull(
         locale,
         input,
         state.initCtx!,
         state.defaultLocale,
         state.originalInput!,
       );
-      state.pullOutput = result;
+      state.pullOutputByLocale.set(locale, result);
 
       return result;
     },
@@ -101,13 +103,25 @@ export function createLoader<I, O, C>(
         throw new Error("Cannot push data without pulling first");
       }
 
+      // Use locale-specific pullInput/pullOutput if available,
+      // otherwise fall back to the default locale's values for backward compatibility
+      // (some loaders push for locales that were never explicitly pulled)
+      const pullInput =
+        state.pullInputByLocale.get(locale) ??
+        state.pullInputByLocale.get(state.defaultLocale) ??
+        null;
+      const pullOutput =
+        state.pullOutputByLocale.get(locale) ??
+        state.pullOutputByLocale.get(state.defaultLocale) ??
+        null;
+
       const pushResult = await lDefinition.push(
         locale,
         data,
         state.originalInput,
         state.defaultLocale,
-        state.pullInput!,
-        state.pullOutput!,
+        pullInput!,
+        pullOutput!,
       );
       return pushResult;
     },

packages/cli/src/cli/loaders/po/index.spec.ts

@@ -406,6 +406,64 @@ msgstr ""`;
     expect(result).not.toContain('"Language: en\\n"');
     expect(result).toContain('msgstr "Hola mundo"');
   });
+
+  it("should preserve Language header for each locale when multiple target locales are pulled before push", async () => {
+    // This test verifies the fix for a bug where pulling multiple target locales
+    // before pushing would cause the Language header to be overwritten with the
+    // wrong locale's value (e.g., es.po would get "Language: en" instead of "Language: es")
+    const loader = createLoader();
+
+    const sourceInput = `msgid ""
+msgstr ""
+"Language: en\\n"
+"Content-Type: text/plain; charset=utf-8\\n"
+
+#: hello.py:1
+msgid "Hello"
+msgstr "Hello"`;
+
+    const spanishInput = `msgid ""
+msgstr ""
+"Language: es\\n"
+"Content-Type: text/plain; charset=utf-8\\n"
+
+#: hello.py:1
+msgid "Hello"
+msgstr ""`;
+
+    const portugueseInput = `msgid ""
+msgstr ""
+"Language: pt\\n"
+"Content-Type: text/plain; charset=utf-8\\n"
+
+#: hello.py:1
+msgid "Hello"
+msgstr ""`;
+
+    await loader.pull("en", sourceInput);
+
+    // Pull multiple target locales (simulates concurrent processing)
+    await loader.pull("es", spanishInput);
+    await loader.pull("pt", portugueseInput);
+
+    const spanishResult = await loader.push("es", {
+      Hello: { singular: "Hola", plural: null },
+    });
+
+    const portugueseResult = await loader.push("pt", {
+      Hello: { singular: "Olá", plural: null },
+    });
+
+    expect(spanishResult).toContain('"Language: es\\n"');
+    expect(spanishResult).not.toContain('"Language: pt\\n"');
+    expect(spanishResult).not.toContain('"Language: en\\n"');
+    expect(spanishResult).toContain('msgstr "Hola"');
+
+    expect(portugueseResult).toContain('"Language: pt\\n"');
+    expect(portugueseResult).not.toContain('"Language: es\\n"');
+    expect(portugueseResult).not.toContain('"Language: en\\n"');
+    expect(portugueseResult).toContain('msgstr "Olá"');
+  });
 });
 
 function createLoader(params: PoLoaderParams = { multiline: false }) {