fix(plugin-gradle): Try to locate the git system config without executing any processes

While it's not possible to match cgit's behavior exactly, this
attempts to read the system gitconfig correcting a regression from 6.21.0 to 6.22.0.

Fixes #2404

Author: zml2008

plugin-gradle/CHANGES.md

@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `3.27.0`).
 
 ## [Unreleased]
+### Fixed
+* Respect system gitconfig when performing git operations ([#2404](https://github.com/diffplug/spotless/issues/2404))
 
 ## [7.0.4] - 2025-05-27
 ### Fixed

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GitRatchetGradle.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2023 DiffPlug
+ * Copyright 2020-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,10 +16,14 @@
 package com.diffplug.gradle.spotless;
 
 import java.io.File;
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
 
 import javax.annotation.Nullable;
 
+import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.Config;
+import org.eclipse.jgit.lib.StoredConfig;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.SystemReader;
@@ -28,20 +32,86 @@
 
 /** Gradle implementation of GitRatchet. */
 public class GitRatchetGradle extends GitRatchet<File> {
+	private static final String[] GIT_EXEC_CANDIDATES = {"git", "git.exe", "git.cmd"};
+
 	static {
-		preventJGitFromCallingExecutables();
+		GitRatchetGradle.redirectJGitExecutions();
 	}
 
-	static void preventJGitFromCallingExecutables() {
-		SystemReader reader = SystemReader.getInstance();
-		SystemReader.setInstance(new DelegatingSystemReader(reader) {
+	static void redirectJGitExecutions() {
+		SystemReader existing = SystemReader.getInstance();
+		SystemReader.setInstance(new DelegatingSystemReader(existing) {
+			private AtomicReference<FileBasedConfig> systemConfig = new AtomicReference<>();
+
+			@Override
+			public StoredConfig getSystemConfig() throws ConfigInvalidException, IOException {
+				FileBasedConfig c = systemConfig.get();
+				if (c == null) {
+					systemConfig.compareAndSet(null,
+							this.openSystemConfig(this.getJGitConfig(), FS.DETECTED));
+					c = systemConfig.get();
+				}
+				updateAll(c);
+				return c;
+			}
+
+			// lifted from SystemReader since it's private
+			private void updateAll(Config config) throws ConfigInvalidException, IOException {
+				if (config == null) {
+					return;
+				}
+
+				updateAll(config.getBaseConfig());
+				if (config instanceof FileBasedConfig) {
+					FileBasedConfig cfg = (FileBasedConfig) config;
+					if (cfg.isOutdated()) {
+						cfg.load();
+					}
+				}
+			}
+
 			@Override
-			public String getenv(String variable) {
-				if ("PATH".equals(variable)) {
-					return "";
-				} else {
-					return super.getenv(variable);
+			public FileBasedConfig openSystemConfig(final Config parent, final FS fs) {
+				// cgit logic: https://git.kernel.org/pub/scm/git/git.git/tree/config.c#n1973 - in git_system_config()
+				// They check the GIT_CONFIG_SYSTEM env var first, then follow up with logic based on compile-time parameters
+				// We can't replicate this exactly so we'll do the closest approximation that Gradle will allow.
+				final String systemPath = this.getenv("GIT_CONFIG_SYSTEM");
+				if (systemPath != null) {
+					fs.setGitSystemConfig(new File(systemPath).getAbsoluteFile());
+					return super.openSystemConfig(parent, fs);
+				}
+
+				// match FS.searchPath
+				File gitExec = null;
+				final String path = this.getenv("PATH");
+				if (path != null) {
+					outer: for (final String p : path.split(File.pathSeparator)) {
+						for (final String name : GIT_EXEC_CANDIDATES) {
+							final File candidate = new File(p, name);
+							if (candidate.isFile() && candidate.canExecute()) {
+								gitExec = candidate.getAbsoluteFile();
+								break outer;
+							}
+						}
+					}
+				}
+
+				// Guess at common locations
+				if (gitExec != null) {
+					// If git exec is at <prefix>/bin/git, this returns <prefix>
+					File prefix = gitExec.getParentFile().getParentFile();
+
+					// Then we try to resolve a config
+					final File systemConfig = new File(prefix, "etc/gitconfig");
+					if (systemConfig.exists()) {
+						fs.setGitSystemConfig(systemConfig);
+						return super.openSystemConfig(parent, fs);
+					}
 				}
+
+				// Fallback to the non-prefixed path (this is not the logic that cgit uses, but oh well)
+				fs.setGitSystemConfig(new File("/etc/gitconfig"));
+				return super.openSystemConfig(parent, fs);
 			}
 		});
 	}
@@ -70,7 +140,7 @@ public String getHostname() {
 
 		@Override
 		public String getenv(String variable) {
-			return reader.getProperty(variable);
+			return reader.getenv(variable);
 		}
 
 		@Override