Add WEB_CONCURRENCY=auto support for automatic CPU detection

Allow --workers and WEB_CONCURRENCY to accept "auto" which resolves
to os.cpu_count(), similar to Puma's approach. Useful for deployments
on platforms that don't automatically set WEB_CONCURRENCY.

Author: davegaeddert

plain/plain/cli/server.py

@@ -1,8 +1,17 @@
+import os
+
 import click
 
 from plain.cli.runtime import without_runtime_setup
 
 
+def parse_workers(ctx: click.Context, param: click.Parameter, value: str) -> int:
+    """Parse workers value - accepts int or 'auto' for CPU count."""
+    if value == "auto":
+        return os.cpu_count() or 1
+    return int(value)
+
+
 @without_runtime_setup
 @click.command()
 @click.option(
@@ -22,10 +31,11 @@
 @click.option(
     "--workers",
     "-w",
-    type=int,
-    default=1,
+    type=str,
+    default="1",
     envvar="WEB_CONCURRENCY",
-    help="Number of worker processes",
+    callback=parse_workers,
+    help="Number of worker processes (or 'auto' for CPU count)",
     show_default=True,
 )
 @click.option(

plain/plain/server/README.md

@@ -25,6 +25,9 @@ By default, the server binds to `127.0.0.1:8000` and uses a single worker proces
 # Run with 4 worker processes
 plain server --workers 4
 
+# Auto-detect based on available CPUs
+plain server --workers auto
+
 # Run with 2 workers and 4 threads each
 plain server --workers 2 --threads 4
 ```
@@ -62,7 +65,7 @@ All options are available via the command line. Run `plain server --help` to see
 | Option             | Default          | Description                                           |
 | ------------------ | ---------------- | ----------------------------------------------------- |
 | `--bind` / `-b`    | `127.0.0.1:8000` | Address to bind (can be used multiple times)          |
-| `--workers` / `-w` | `1`              | Number of worker processes                            |
+| `--workers` / `-w` | `1`              | Number of worker processes (or `auto` for CPU count)  |
 | `--threads`        | `1`              | Number of threads per worker                          |
 | `--timeout` / `-t` | `30`             | Worker timeout in seconds                             |
 | `--reload`         | `False`          | Restart workers when code changes                     |
@@ -76,11 +79,11 @@ All options are available via the command line. Run `plain server --help` to see
 
 ## Environment variables
 
-| Variable              | Description                                                          |
-| --------------------- | -------------------------------------------------------------------- |
-| `WEB_CONCURRENCY`     | Sets the number of workers (commonly used by Heroku and other PaaS)  |
-| `SENDFILE`            | Enable sendfile() syscall (`1`, `yes`, `true`, or `y` to enable)     |
-| `FORWARDED_ALLOW_IPS` | Comma-separated list of trusted proxy IPs (default: `127.0.0.1,::1`) |
+| Variable              | Description                                                              |
+| --------------------- | ------------------------------------------------------------------------ |
+| `WEB_CONCURRENCY`     | Sets the number of workers (use `auto` to detect CPU cores, or a number) |
+| `SENDFILE`            | Enable sendfile() syscall (`1`, `yes`, `true`, or `y` to enable)         |
+| `FORWARDED_ALLOW_IPS` | Comma-separated list of trusted proxy IPs (default: `127.0.0.1,::1`)     |
 
 ## Signals