Makes tools optional for experimental_shell_command (pantsbuild#1…

…7704)

This change looks for the `bash` exit code 127, which indicates that a command could not be found. An error log is raised at that point.

As a side effect, we no longer need to nag the user to set `tools`, which makes `experimental_shell_commands` that depend only on `bash` internal commands a bit more concise.

Note that this would _not_ raise a warning if the error is not one that bash would ignore -- e.g. running a command in backticks, without `set -e` enabled.

src/python/pants/backend/shell/target_types.py

@@ -290,7 +290,7 @@ class ShellCommandTimeoutField(IntField):
 
 class ShellCommandToolsField(StringSequenceField):
     alias = "tools"
-    required = True
+    default = ()
     help = softwrap(
         """
         Specify required executable tools that might be used.

src/python/pants/backend/shell/util_rules/shell_command.py

@@ -44,7 +44,7 @@
     MergeDigests,
     Snapshot,
 )
-from pants.engine.process import Process, ProcessResult
+from pants.engine.process import FallibleProcessResult, Process, ProcessResult, ProductDescription
 from pants.engine.rules import Get, MultiGet, collect_rules, rule, rule_helper
 from pants.engine.target import (
     FieldSetsPerTarget,
@@ -137,7 +137,7 @@ async def _prepare_process_request_from_target(shell_command: Target) -> ShellCo
     output_directories = tuple(d for d in outputs if d.endswith("/"))
 
     return ShellCommandProcessRequest(
-        description="the `{alias}` at {address}",
+        description=f"the `{shell_command.alias}` at `{shell_command.address}`",
         interactive=interactive,
         working_directory=working_directory,
         command=command,
@@ -173,8 +173,9 @@ async def run_shell_command(
     environment_name = await Get(
         EnvironmentName, EnvironmentNameRequest, EnvironmentNameRequest.from_target(shell_command)
     )
-    result = await Get(
-        ProcessResult,
+
+    fallible_result = await Get(
+        FallibleProcessResult,
         {
             environment_name: EnvironmentName,
             ShellCommandProcessFromTargetRequest(
@@ -183,6 +184,23 @@ async def run_shell_command(
         },
     )
 
+    if fallible_result.exit_code == 127:
+        logger.error(
+            f"`{shell_command.alias}` requires the names of any external commands used by this "
+            f"shell command to be specified in the `{ShellCommandToolsField.alias}` field. If "
+            f"`bash` cannot find a tool, add it to the `{ShellCommandToolsField.alias}` field."
+        )
+
+    result = await Get(
+        ProcessResult,
+        {
+            fallible_result: FallibleProcessResult,
+            ProductDescription(
+                f"the `{shell_command.alias}` at `{shell_command.address}`"
+            ): ProductDescription,
+        },
+    )
+
     if shell_command[ShellCommandLogOutputField].value:
         if result.stdout:
             logger.info(result.stdout.decode())
@@ -253,9 +271,6 @@ async def prepare_shell_command_process(
             "CHROOT": "{chroot}",
         }
     else:
-        if not tools:
-            raise ValueError(f"Must provide any `tools` used by the {description}.")
-
         resolved_tools = await _shell_command_tools(shell_setup, tools, f"execute {description}")
         tools = tuple(tool for tool in sorted(resolved_tools))