fix: isolate per-task runner envs (#959)

## Summary
- clone the runner environment map for each task before injecting runtime and OIDC tokens
- keep the shared base environment immutable so concurrent jobs cannot hit `concurrent map writes`
- add a unit test covering task-local env cloning

Fixes #958

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/959
Reviewed-by: Nicolas <bircni@icloud.com>

internal/app/run/runner.go

@@ -218,6 +218,14 @@ func (r *Runner) Run(ctx context.Context, task *runnerv1.Task) error {
 	return nil
 }
 
+func (r *Runner) cloneEnvs() map[string]string {
+	// +3 reserves space for the per-task keys injected by run():
+	// ACTIONS_ID_TOKEN_REQUEST_URL, ACTIONS_ID_TOKEN_REQUEST_TOKEN, ACTIONS_RUNTIME_TOKEN.
+	envs := make(map[string]string, len(r.envs)+3)
+	maps.Copy(envs, r.envs)
+	return envs
+}
+
 // getDefaultActionsURL
 // when DEFAULT_ACTIONS_URL == "https://github.com" and GithubMirror is not blank,
 // it should be set to GithubMirror first.
@@ -251,6 +259,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 	reporter.ResetSteps(len(job.Steps))
 
 	taskContext := task.Context.Fields
+	envs := r.cloneEnvs()
 
 	log.Infof("task %v repo is %v %v %v", task.Id, taskContext["repository"].GetStringValue(),
 		r.getDefaultActionsURL(task),
@@ -281,9 +290,9 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 	}
 
 	if actionsIDTokenRequestURL := taskContext["actions_id_token_request_url"].GetStringValue(); actionsIDTokenRequestURL != "" {
-		r.envs["ACTIONS_ID_TOKEN_REQUEST_URL"] = actionsIDTokenRequestURL
-		r.envs["ACTIONS_ID_TOKEN_REQUEST_TOKEN"] = taskContext["actions_id_token_request_token"].GetStringValue()
-		task.Secrets["ACTIONS_ID_TOKEN_REQUEST_TOKEN"] = r.envs["ACTIONS_ID_TOKEN_REQUEST_TOKEN"]
+		envs["ACTIONS_ID_TOKEN_REQUEST_URL"] = actionsIDTokenRequestURL
+		envs["ACTIONS_ID_TOKEN_REQUEST_TOKEN"] = taskContext["actions_id_token_request_token"].GetStringValue()
+		task.Secrets["ACTIONS_ID_TOKEN_REQUEST_TOKEN"] = envs["ACTIONS_ID_TOKEN_REQUEST_TOKEN"]
 	}
 
 	giteaRuntimeToken := taskContext["gitea_runtime_token"].GetStringValue()
@@ -291,7 +300,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		// use task token to action api token for previous Gitea Server Versions
 		giteaRuntimeToken = preset.Token
 	}
-	r.envs["ACTIONS_RUNTIME_TOKEN"] = giteaRuntimeToken
+	envs["ACTIONS_RUNTIME_TOKEN"] = giteaRuntimeToken
 	// Mask the runtime token so it cannot be echoed in user step output; it is
 	// now also the cache server's bearer credential and leaking it would let
 	// any reader of the log impersonate this job against the cache.
@@ -344,7 +353,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		ForceRebuild:          r.cfg.Container.ForceRebuild,
 		LogOutput:             true,
 		JSONLogger:            false,
-		Env:                   r.envs,
+		Env:                   envs,
 		Secrets:               task.Secrets,
 		GitHubInstance:        strings.TrimSuffix(r.client.Address(), "/"),
 		AutoRemove:            true,

internal/app/run/runner_env_test.go

@@ -0,0 +1,61 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package run
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRunnerCloneEnvsReturnsTaskLocalCopy(t *testing.T) {
+	r := &Runner{
+		envs: map[string]string{
+			"ACTIONS_CACHE_URL":   "http://cache.example",
+			"ACTIONS_RUNTIME_URL": "http://runner.example",
+		},
+	}
+
+	cloned := r.cloneEnvs()
+	require.Equal(t, r.envs, cloned)
+
+	cloned["ACTIONS_RUNTIME_TOKEN"] = "task-token"
+	cloned["ACTIONS_ID_TOKEN_REQUEST_URL"] = "http://oidc.example"
+
+	assert.NotContains(t, r.envs, "ACTIONS_RUNTIME_TOKEN")
+	assert.NotContains(t, r.envs, "ACTIONS_ID_TOKEN_REQUEST_URL")
+	assert.Equal(t, "http://cache.example", r.envs["ACTIONS_CACHE_URL"])
+}
+
+// Regression test for #958: concurrent tasks writing task-specific env keys
+// used to race on the shared r.envs map and crash the runner with
+// "fatal error: concurrent map writes". Each task must mutate its own clone.
+func TestRunnerCloneEnvsConcurrentMutation(t *testing.T) {
+	r := &Runner{
+		envs: map[string]string{
+			"ACTIONS_CACHE_URL":   "http://cache.example",
+			"ACTIONS_RUNTIME_URL": "http://runner.example",
+		},
+	}
+
+	const goroutines = 16
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+	for range goroutines {
+		go func() {
+			defer wg.Done()
+			envs := r.cloneEnvs()
+			envs["ACTIONS_RUNTIME_TOKEN"] = "task-token"
+			envs["ACTIONS_ID_TOKEN_REQUEST_URL"] = "http://oidc.example"
+			envs["ACTIONS_ID_TOKEN_REQUEST_TOKEN"] = "oidc-token"
+		}()
+	}
+	wg.Wait()
+
+	assert.NotContains(t, r.envs, "ACTIONS_RUNTIME_TOKEN")
+	assert.NotContains(t, r.envs, "ACTIONS_ID_TOKEN_REQUEST_URL")
+	assert.NotContains(t, r.envs, "ACTIONS_ID_TOKEN_REQUEST_TOKEN")
+}