Support execution on Windows by using ssh-agent provided by git

pom.xml

@@ -89,6 +89,10 @@
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>ssh-credentials</artifactId>
     </dependency>
+    <dependency>
+      <groupId>org.jenkins-ci.plugins</groupId>
+      <artifactId>git-client</artifactId>
+    </dependency>
     <!-- jenkins dependencies -->
     <!-- test dependencies -->
     <dependency>

src/main/java/com/cloudbees/jenkins/plugins/sshagent/exec/ExecRemoteAgent.java

@@ -24,19 +24,26 @@
 
 package com.cloudbees.jenkins.plugins.sshagent.exec;
 
-import hudson.AbortException;
-import hudson.FilePath;
-import hudson.Launcher;
-import hudson.model.TaskListener;
-import hudson.slaves.WorkspaceList;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.Serializable;
 import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 
+import hudson.AbortException;
+import hudson.FilePath;
+import hudson.Functions;
+import hudson.Launcher;
+import hudson.model.TaskListener;
+import hudson.plugins.git.GitTool;
+import hudson.slaves.WorkspaceList;
+
 /**
  * Runs a native SSH agent installed on a system.
  */
@@ -46,15 +53,46 @@
 
     /** Agent environment used for {@code ssh-add} and {@code ssh-agent -k}. */
     private final Map<String, String> agentEnv;
+    private final String sshAgentExe;
 
     public ExecRemoteAgent(Launcher launcher, TaskListener listener) throws IOException, InterruptedException {
+        sshAgentExe = !Functions.isWindows() ? "ssh-agent" : searchSSHAgentExeForWindows(launcher, listener);
+        String agentOutput = executeCommand(launcher, listener, sshAgentExe);
+        agentEnv = parseAgentEnv(agentOutput, listener); // TODO could include local filenames, better to look up remote charset
+    }
+
+    private static String executeCommand(Launcher launcher, TaskListener listener, String... cmd)
+            throws IOException, InterruptedException, AbortException {
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
-        if (launcher.launch().cmds("ssh-agent").stdout(baos).start()
-                .joinWithTimeout(1, TimeUnit.MINUTES, listener) != 0) {
+        if (launcher.launch().cmds(cmd).stdout(baos).start().joinWithTimeout(1, TimeUnit.MINUTES, listener) != 0) {
             String reason = new String(baos.toByteArray(), StandardCharsets.US_ASCII);
             throw new AbortException("Failed to run ssh-agent: " + reason);
         }
-        agentEnv = parseAgentEnv(new String(baos.toByteArray(), StandardCharsets.US_ASCII), listener); // TODO could include local filenames, better to look up remote charset
+        return new String(baos.toByteArray(), StandardCharsets.US_ASCII);
+    }
+
+    private static final Path GIT_SSH_AGENT_PATH_WINDOWS = Path.of("usr", "bin", "ssh-agent.exe");
+
+    private static String searchSSHAgentExeForWindows(Launcher launcher, TaskListener listener)
+            throws IOException, InterruptedException {
+        Optional<Path> defaultGitHome = Optional.ofNullable(GitTool.getDefaultInstallation()) //
+                .map(GitTool::getHome).map(Path::of);
+        if (defaultGitHome.isPresent()) {
+            Path sshAgentExe = defaultGitHome.get().resolve(GIT_SSH_AGENT_PATH_WINDOWS);
+            if (Files.isRegularFile(sshAgentExe)) {
+                return sshAgentExe.toString();
+            }
+        }
+        String gitPaths = executeCommand(launcher, listener, "where", "git");
+        List<Path> paths = gitPaths.lines().map(Path::of).toList();
+        for (Path gitExe : paths) {
+            Path sshAgentExe = gitExe.getParent().getParent().resolve(GIT_SSH_AGENT_PATH_WINDOWS);
+            if (Files.exists(sshAgentExe)) {
+                return sshAgentExe.toString();
+            }
+        }
+        throw new IllegalStateException(
+                "Executing with default ssh-agent on Windows is not supported and an alternative implementation from a git installation is not available.");
     }
 
     /**
@@ -84,10 +122,10 @@
                     env.put("DISPLAY", "bogus"); // just to force using SSH_ASKPASS
                     env.put("SSH_ASKPASS", askpass.getRemote());
                 }
-                
+
                 // as the next command is in quiet mode, we just add a message to the log
                 listener.getLogger().println("Running ssh-add (command line suppressed)");
-                
+
                 if (launcher.launch().quiet(true).cmds("ssh-add", keyFile.getRemote()).envs(env)
                         .stdout(listener).start().joinWithTimeout(1, TimeUnit.MINUTES, listener) != 0) {
                     throw new AbortException("Failed to run ssh-add");
@@ -117,7 +155,7 @@
             throw new AbortException("Failed to run ssh-agent -k");
         }
     }
-    
+
     /**
      * Parses ssh-agent output.
      */
@@ -133,10 +171,10 @@
         // get SSH_AGENT_PID
         env.put(AgentPidVar, getAgentValue(agentOutput, AgentPidVar));
         listener.getLogger().println(AgentPidVar + "=" + env.get(AgentPidVar));
-        
+
         return env;
     }
-    
+
     /**
      * Parses a value from ssh-agent output.
      */
@@ -145,16 +183,16 @@
         int end = agentOutput.indexOf(';', pos);
         return agentOutput.substring(pos, end);
     }
-    
+
     /**
-     * Creates a self-deleting script for SSH_ASKPASS. Self-deleting to be able to detect a wrong passphrase. 
+     * Creates a self-deleting script for SSH_ASKPASS. Self-deleting to be able to detect a wrong passphrase.
      */
     private FilePath createAskpassScript(FilePath temp) throws IOException, InterruptedException {
         // TODO: assuming that ssh-add runs the script in shell even on Windows, not cmd
         //       for cmd following could work
         //       suffix = ".bat";
         //       script = "@ECHO %SSH_PASSPHRASE%\nDEL \"" + askpass.getAbsolutePath() + "\"\n";
-        
+
         FilePath askpass = temp.createTextTempFile("askpass_", ".sh", "#!/bin/sh\necho \"$SSH_PASSPHRASE\"\nrm \"$0\"\n");
 
         // executable only for a current user