FS: Allow gitPrefix to be set and cached

This permits callers to modify the meaning of gitPrefix, which
may be useful if their application allows the user to select
the location where C Git is installed.

Bug: 337101
Change-Id: I07362a5772da4955e01406bdeb8eaf87416be1d6
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -101,6 +101,8 @@ public abstract class FS {
 
 	private final File userHome;
 
+	private volatile Holder<File> gitPrefix;
+
 	/**
 	 * Constructs a file system abstraction.
 	 */
@@ -258,7 +260,29 @@ public abstract class FS {
 	}
 
 	/** @return the $prefix directory C Git would use. */
-	public abstract File gitPrefix();
+	public File gitPrefix() {
+		Holder<File> p = gitPrefix;
+		if (p == null) {
+			p = new Holder<File>(discoverGitPrefix());
+			gitPrefix = p;
+		}
+		return p.value;
+	}
+
+	/** @return the $prefix directory C Git would use. */
+	protected abstract File discoverGitPrefix();
+
+	/**
+	 * Set the $prefix directory C Git uses.
+	 *
+	 * @param path
+	 *            the directory. Null if C Git is not installed.
+	 * @return {@code this}
+	 */
+	public FS setGitPrefix(File path) {
+		gitPrefix = new Holder<File>(path);
+		return this;
+	}
 
 	/**
 	 * Initialize a ProcesssBuilder to run a command using the system shell.
@@ -273,4 +297,12 @@ public abstract class FS {
 	 *         populating directory, environment, and then start the process.
 	 */
 	public abstract ProcessBuilder runInShell(String cmd, String[] args);
+
+	private static class Holder<V> {
+		final V value;
+
+		Holder(V value) {
+			this.value = value;
+		}
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java

@@ -52,7 +52,7 @@ import java.util.List;
 
 abstract class FS_POSIX extends FS {
 	@Override
-	public File gitPrefix() {
+	protected File discoverGitPrefix() {
 		String path = SystemReader.getInstance().getenv("PATH");
 		File gitExe = searchPath(path, "git");
 		if (gitExe != null)

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_Win32.java

@@ -64,9 +64,6 @@ class FS_Win32 extends FS {
 				&& StringUtils.toLowerCase(osDotName).indexOf("windows") != -1;
 	}
 
-	private File gitPrefix;
-	private boolean gitPrefixEvaluated;
-
 	public boolean supportsExecute() {
 		return false;
 	}
@@ -85,15 +82,12 @@ class FS_Win32 extends FS {
 	}
 
 	@Override
-	public File gitPrefix() {
-		if (gitPrefixEvaluated)
-			return gitPrefix;
-
+	protected File discoverGitPrefix() {
 		String path = SystemReader.getInstance().getenv("PATH");
 		File gitExe = searchPath(path, "git.exe", "git.cmd");
 		if (gitExe != null)
-			gitPrefix = gitExe.getParentFile().getParentFile();
-		else {
+			return gitExe.getParentFile().getParentFile();
+
 		// This isn't likely to work, if bash is in $PATH, git should
 		// also be in $PATH. But its worth trying.
 		//
@@ -101,11 +95,8 @@ class FS_Win32 extends FS {
 				new String[] { "bash", "--login", "-c", "which git" }, //
 				Charset.defaultCharset().name());
 		if (w != null)
-				gitPrefix = new File(w).getParentFile().getParentFile();
-		}
-
-		gitPrefixEvaluated = true;
-		return gitPrefix;
+			return new File(w).getParentFile().getParentFile();
+		return null;
 	}
 
 	@Override