Make environment-to-ini support loading key value from file (#24832)

Replace #19857

Close #19856
Close #10311
Close #10123

Major changes:

1. Move a lot of code from `environment-to-ini.go` to `config_env.go` to
make them testable.
2. Add `__FILE` support
3. Update documents
4. Add tests

modules/setting/config_env_test.go

@@ -0,0 +1,97 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"gopkg.in/ini.v1"
+)
+
+func TestDecodeEnvSectionKey(t *testing.T) {
+	ok, section, key := decodeEnvSectionKey("SEC__KEY")
+	assert.True(t, ok)
+	assert.Equal(t, "sec", section)
+	assert.Equal(t, "KEY", key)
+
+	ok, section, key = decodeEnvSectionKey("sec__key")
+	assert.True(t, ok)
+	assert.Equal(t, "sec", section)
+	assert.Equal(t, "key", key)
+
+	ok, section, key = decodeEnvSectionKey("LOG_0x2E_CONSOLE__STDERR")
+	assert.True(t, ok)
+	assert.Equal(t, "log.console", section)
+	assert.Equal(t, "STDERR", key)
+
+	ok, section, key = decodeEnvSectionKey("SEC")
+	assert.False(t, ok)
+	assert.Equal(t, "", section)
+	assert.Equal(t, "", key)
+}
+
+func TestDecodeEnvironmentKey(t *testing.T) {
+	prefix := "GITEA__"
+	suffix := "__FILE"
+
+	ok, section, key, file := decodeEnvironmentKey(prefix, suffix, "SEC__KEY")
+	assert.False(t, ok)
+	assert.Equal(t, "", section)
+	assert.Equal(t, "", key)
+	assert.False(t, file)
+
+	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA__SEC")
+	assert.False(t, ok)
+	assert.Equal(t, "", section)
+	assert.Equal(t, "", key)
+	assert.False(t, file)
+
+	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA__SEC__KEY")
+	assert.True(t, ok)
+	assert.Equal(t, "sec", section)
+	assert.Equal(t, "KEY", key)
+	assert.False(t, file)
+
+	// with "__FILE" suffix, it doesn't support to write "[sec].FILE" to config (no such key FILE is used in Gitea)
+	// but it could be fixed in the future by adding a new suffix like "__VALUE" (no such key VALUE is used in Gitea either)
+	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA__SEC__FILE")
+	assert.False(t, ok)
+	assert.Equal(t, "", section)
+	assert.Equal(t, "", key)
+	assert.True(t, file)
+
+	ok, section, key, file = decodeEnvironmentKey(prefix, suffix, "GITEA__SEC__KEY__FILE")
+	assert.True(t, ok)
+	assert.Equal(t, "sec", section)
+	assert.Equal(t, "KEY", key)
+	assert.True(t, file)
+}
+
+func TestEnvironmentToConfig(t *testing.T) {
+	cfg := ini.Empty()
+
+	changed := EnvironmentToConfig(cfg, "GITEA__", "__FILE", nil)
+	assert.False(t, changed)
+
+	cfg, err := ini.Load([]byte(`
+[sec]
+key = old
+`))
+	assert.NoError(t, err)
+
+	changed = EnvironmentToConfig(cfg, "GITEA__", "__FILE", []string{"GITEA__sec__key=new"})
+	assert.True(t, changed)
+	assert.Equal(t, "new", cfg.Section("sec").Key("key").String())
+
+	changed = EnvironmentToConfig(cfg, "GITEA__", "__FILE", []string{"GITEA__sec__key=new"})
+	assert.False(t, changed)
+
+	tmpFile := t.TempDir() + "/the-file"
+	_ = os.WriteFile(tmpFile, []byte("value-from-file"), 0o644)
+	changed = EnvironmentToConfig(cfg, "GITEA__", "__FILE", []string{"GITEA__sec__key__FILE=" + tmpFile})
+	assert.True(t, changed)
+	assert.Equal(t, "value-from-file", cfg.Section("sec").Key("key").String())
+}