Support resizing output image

When the final image does not contain the rootfs it must be grown before
being able to boot it. Since the exact size to grow is not easily known
to the user, it's best if the image is already grown to the right size.
Also, when flashing to disk it helps to have the image be the minimum
size needed because otherwise flashing on a too small disk succeeds but
boot will fail. With the right image size it's easier for the user to
know whether the disk is large enough.

Author: pothos

mkosi/__init__.py

@@ -4129,6 +4129,17 @@ def build_image(context: Context) -> None:
     elif context.config.output_format == OutputFormat.directory:
         context.root.rename(context.staging / context.config.output_with_format)
 
+    if context.config.output_size and context.config.output_format == OutputFormat.disk:
+        output = context.staging / context.config.output_with_format
+        current_size = output.stat().st_size
+        if context.config.output_size > current_size:
+            with complete_step(f"Resizing output image to {format_bytes(context.config.output_size)}…"):
+                os.truncate(output, context.config.output_size)
+        else:
+            log_notice(
+                f"Can't change the output size because the image is already larger ({format_bytes(current_size)}) than requested"  # noqa: E501
+            )
+
     if context.config.output_format.use_outer_compression():
         maybe_compress(
             context,

mkosi/config.py

@@ -2023,6 +2023,7 @@ class Config:
     manifest_format: list[ManifestFormat]
     output: str
     output_extension: str
+    output_size: Optional[int]
     compress_output: Compression
     compress_level: int
     output_dir: Optional[Path]
@@ -2832,6 +2833,13 @@ def parse_kernel_module_filter_regexp(p: str) -> str:
         default_factory=lambda ns: ns["output_format"].extension(),
         default_factory_depends=("output_format",),
     ),
+    ConfigSetting(
+        dest="output_size",
+        metavar="SIZE",
+        section="Output",
+        parse=config_parse_bytes,
+        help="Resize the output image to this size",
+    ),
     ConfigSetting(
         dest="compress_output",
         metavar="ALG",
@@ -5627,6 +5635,7 @@ def summary(config: Config) -> str:
                       Output Format: {config.output_format}
                    Manifest Formats: {maniformats}
                              Output: {bold(config.output_with_compression)}
+                        Output Size: {format_bytes_or_none(config.output_size)}
                         Compression: {config.compress_output}
                   Compression Level: {config.compress_level}
                    Output Directory: {config.output_dir_or_cwd()}

mkosi/resources/man/mkosi.1.md

@@ -644,6 +644,12 @@ boolean argument: either `1`, `yes`, or `true` to enable, or `0`, `no`,
     any compression extension which will be appended to this extension if compression
     is enabled.
 
+`OutputSize=`, `--output-size=`
+:   If set, the output disk image is resized to the given size. Takes a size in
+    bytes. The suffixes `K`, `M` and `G` can be used to specify the size in kilobytes,
+    megabytes and gigabytes. This setting only grows the disk and is limited to full
+    disk images and no other output format.
+
 `CompressOutput=`, `--compress-output=`
 :   Configure compression for the resulting image or archive. The argument can be
     either a boolean or a compression algorithm (**xz**, **zstd**). **zstd**

tests/test_json.py

@@ -268,6 +268,7 @@ def test_config() -> None:
             "OutputDirectory": "/your/output/here",
             "OutputExtension": "raw",
             "OutputMode": 83,
+            "OutputSize": null,
             "Overlay": true,
             "PackageCacheDirectory": "/a/b/c",
             "PackageDirectories": [],
@@ -519,6 +520,7 @@ def test_config() -> None:
         openpgp_tool="gpg",
         output_dir=Path("/your/output/here"),
         output_extension="raw",
+        output_size=None,
         output_format=OutputFormat.uki,
         output_mode=0o123,
         output="outfile",