Introduce `fs.read` to read a file as a string

Following #7890, this patch introduces the ability to read the contents
of a file to the fs module.

This patch introduces the ability to read files at configure time, but
has some restrictions:
    - binary files are not supported (I don't think this will prove a
    problem, and if people are wanting to do something with binary
    files, they should probably be shelling out to their own script).
    - Only files outside the build directory allowed. This limitation
      should prevent build loops.
Given that reading an arbitrary file at configure time can affect the
configuration in almost arbitrary ways, meson should force a reconfigure
when the given file changes. This is non-configurable, but this can
easily be changed with a future keyword argument.

1 files changed, 64 insertions, 1 deletions

diff --git a/mesonbuild/modules/fs.py b/mesonbuild/modules/fs.py
index 2ff256b..44986f8 100644
--- a/mesonbuild/modules/fs.py
+++ b/mesonbuild/modules/fs.py
@@ -14,18 +14,25 @@
 
 import typing as T
 import hashlib
+import os
 from pathlib import Path, PurePath, PureWindowsPath
 
 from .. import mlog
 from . import ExtensionModule
 from . import ModuleReturnValue
-from ..mesonlib import MesonException
+from ..mesonlib import (
+    File,
+    FileOrString,
+    MesonException,
+    path_is_in_root,
+)
 from ..interpreterbase import FeatureNew
 
 from ..interpreterbase import stringArgs, noKwargs
 if T.TYPE_CHECKING:
     from ..interpreter import Interpreter, ModuleState
 
+
 class FSModule(ExtensionModule):
 
     def __init__(self, interpreter: 'Interpreter') -> None:
@@ -193,5 +200,61 @@ class FSModule(ExtensionModule):
         new = original.stem
         return ModuleReturnValue(str(new), [])
 
+    #@permittedKwargs({'encoding'})
+    @FeatureNew('fs.read', '0.57.0')
+    def read(
+        self,
+        state: 'ModuleState',
+        args: T.Sequence['FileOrString'],
+        kwargs: T.Dict[str, T.Any]
+    ) -> ModuleReturnValue:
+        '''
+        Read a file from the source tree and return its value as a decoded
+        string. If the encoding is not specified, the file is assumed to be
+        utf-8 encoded. Paths must be relative by default (to prevent accidents)
+        and are forbidden to be read from the build directory (to prevent build
+        loops)
+        '''
+        if len(args) != 1:
+            raise MesonException('expected single positional <path> arg')
+
+        path = args[0]
+        if not isinstance(path, (str, File)):
+            raise MesonException(
+                '<path> positional argument must be string or files() object')
+
+        encoding = kwargs.get('encoding', 'utf-8')
+        if not isinstance(encoding, str):
+            raise MesonException('`encoding` parameter must be a string')
+
+        src_dir = self.interpreter.environment.source_dir
+        sub_dir = self.interpreter.subdir
+        build_dir = self.interpreter.environment.get_build_dir()
+
+        if isinstance(path, File):
+            if path.is_built:
+                raise MesonException(
+                    'fs.read_file does not accept built files() objects')
+            path = os.path.join(src_dir, path.relative_name())
+        else:
+            if sub_dir:
+                src_dir = os.path.join(src_dir, sub_dir)
+            path = os.path.join(src_dir, path)
+
+        path = os.path.abspath(path)
+        if path_is_in_root(Path(path), Path(build_dir), resolve=True):
+            raise MesonException('path must not be in the build tree')
+        try:
+            with open(path, 'r', encoding=encoding) as f:
+                data = f.read()
+        except UnicodeDecodeError:
+            raise MesonException(f'decoding failed for {path}')
+        # Reconfigure when this file changes as it can contain data used by any
+        # part of the build configuration (e.g. `project(..., version:
+        # fs.read_file('VERSION')` or `configure_file(...)`
+        self.interpreter.add_build_def_file(path)
+        return ModuleReturnValue(data, [])
+
+
 def initialize(*args: T.Any, **kwargs: T.Any) -> FSModule:
     return FSModule(*args, **kwargs)