Basic zsh completion support

- This adds basic zsh support similarly to
  existing bash completion
- New command "completion zsh"
- Fix internal recursive command completion model for
  cases with deep nested commands
- Fixes #927

spring-shell-standard/src/main/java/org/springframework/shell/standard/completion/AbstractCompletions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 the original author or authors.
+ * Copyright 2022-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -88,8 +88,9 @@ public abstract class AbstractCompletions {
 					else {
 						commandKey = splitKeys[i];
 					}
+					String desc = i + 1 < splitKeys.length ? null : registration.getDescription();
 					DefaultCommandModelCommand command = commands.computeIfAbsent(commandKey,
-							(fullCommand) -> new DefaultCommandModelCommand(fullCommand, main));
+							(fullCommand) -> new DefaultCommandModelCommand(fullCommand, main, desc));
 
 					// TODO long vs short
 					List<CommandModelOption> options = registration.getOptions().stream()
@@ -147,8 +148,13 @@ public abstract class AbstractCompletions {
 	interface CommandModelCommand  {
 
 		/**
-		 * Gets sub-commands known to this command.
+		 * Gets a description of a command.
+		 * @return command description
+		 */
+		String getDescription();
 
+		/**
+		 * Gets sub-commands known to this command.
 		 * @return known sub-commands
 		 */
 		List<CommandModelCommand> getCommands();
@@ -240,12 +246,19 @@ public abstract class AbstractCompletions {
 
 		private String fullCommand;
 		private String mainCommand;
+		private String description;
 		private List<CommandModelCommand> commands = new ArrayList<>();
 		private List<CommandModelOption> options = new ArrayList<>();
 
-		DefaultCommandModelCommand(String fullCommand, String mainCommand) {
+		DefaultCommandModelCommand(String fullCommand, String mainCommand, String description) {
 			this.fullCommand = fullCommand;
 			this.mainCommand = mainCommand;
+			this.description = description;
+		}
+
+		@Override
+		public String getDescription() {
+			return description;
 		}
 
 		@Override
@@ -293,6 +306,9 @@ public abstract class AbstractCompletions {
 		}
 
 		void addCommand(DefaultCommandModelCommand command) {
+			if (commands.contains(command)) {
+				return;
+			}
 			commands.add(command);
 		}
 

spring-shell-standard/src/main/java/org/springframework/shell/standard/completion/ZshCompletions.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.standard.completion;
+
+import org.springframework.core.io.ResourceLoader;
+import org.springframework.shell.command.CommandCatalog;
+
+/**
+ * Completion script generator for a {@code zsh}.
+ *
+ * @author Janne Valkealahti
+ */
+public class ZshCompletions extends AbstractCompletions {
+
+	public ZshCompletions(ResourceLoader resourceLoader, CommandCatalog commandCatalog) {
+		super(resourceLoader, commandCatalog);
+	}
+
+	public String generate(String rootCommand) {
+		CommandModel model = generateCommandModel();
+		return builder()
+				.attribute("name", rootCommand)
+				.attribute("model", model)
+				.group("classpath:completion/zsh.stg")
+				.appendGroup("main")
+				.build();
+	}
+}

spring-shell-standard/src/main/resources/completion/zsh.stg

@@ -0,0 +1,89 @@
+//
+// pre content template before commands
+// needs to escape some > characters
+//
+pre(name) ::= <<
+#compdef _<name> <name>
+>>
+
+//
+// commands section with command and description
+//
+cmd_and_desc(command) ::= <<
+"<command.mainCommand>:<command.description>"
+>>
+
+//
+// case for command to call function
+//
+cmd_func(name,command) ::= <<
+<command.mainCommand>)
+  _<name>_<command.commandParts:{p | <p>}; separator="_">
+  ;;
+>>
+
+//
+// recursive sub commands
+//
+sub_command(name,command,commands) ::= <<
+function _<name>_<command.commandParts:{p | <p>}; separator="_"> {
+  local -a commands
+
+  _arguments -C \
+    <command.flags:{f | "<f>" \\}; separator="\n">
+    "1: :->cmnds" \
+    "*::arg:->args"
+
+  case $state in
+  cmnds)
+    commands=(
+      <commands:{c | <cmd_and_desc(c)>}; separator="\n">
+    )
+    _describe "command" commands
+    ;;
+  esac
+
+  case "$words[1]" in
+  <commands:{c | <cmd_func(name,c)>}; separator="\n">
+  esac
+}
+
+<commands:{c | <sub_command(name,c,c.commands)>}; separator="\n\n">
+>>
+
+//
+// top level commands
+//
+top_commands(name,commands) ::= <<
+function _<name> {
+  local -a commands
+
+  _arguments -C \
+    "1: :->cmnds" \
+    "*::arg:->args"
+
+  case $state in
+  cmnds)
+    commands=(
+      <commands:{c | <cmd_and_desc(c)>}; separator="\n">
+    )
+    _describe "command" commands
+    ;;
+  esac
+
+  case "$words[1]" in
+  <commands:{c | <cmd_func(name,c)>}; separator="\n">
+  esac
+}
+
+<commands:{c | <sub_command(name,c,c.commands)>}; separator="\n\n">
+>>
+
+//
+// main template to call from render
+//
+main(name, model) ::= <<
+<pre(name)>
+
+<top_commands(name,model.commands)>
+>>