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, 17 insertions, 0 deletions

diff --git a/docs/markdown/howtox.md b/docs/markdown/howtox.md
index f05f3e8..8c8c0c0 100644
--- a/docs/markdown/howtox.md
+++ b/docs/markdown/howtox.md
@@ -139,6 +139,23 @@ cdata.set('SOMETHING', txt)
 configure_file(...)
 ```
 
+## Generate configuration data from files
+
+`The [fs module](#Fs-modules) offers the `read` function` which enables adding
+the contents of arbitrary files to configuration data (among other uses):
+
+```meson
+fs = import('fs')
+cdata = configuration_data()
+copyright = fs.read('LICENSE')
+cdata.set('COPYRIGHT', copyright)
+if build_machine.system() == 'linux'
+    os_release = fs.read('/etc/os-release')
+    cdata.set('LINUX_BUILDER', os_release)
+endif
+configure_file(...)
+```
+
 ## Generate a runnable script with `configure_file`
 
 `configure_file` preserves metadata so if your template file has