backends: limit maximum path of generated filenames

When calculating the output filename for a compiled object, we sanitize
the whole input path, more or less. In cases where the input path is
very long, this can overflow the max length of an individual filename
component.

At the same time, we do want unique names so people can recognize what
these outputs actually are. Compromise:

- for filepaths with >5 components (which are a lot more likely to cause
  problems, and simultanously less likely to have crucial information that
  far back in the filepath)
- if an sha1 hash of the full path, replacing all *but* those last 5
  components, produces a path that is *shorter* than the original path

... then use that modified path canonicalization via a hash. Due to the
use of hashes, it's unique enough to guarantee correct builds. Because
we keep the last 5 components intact, it's easy to tell what the output
file is compiled from.

Fixes building in ecosystems such as spack, where the build environment
is a very long path containing repetitions of
`__spack_path_placeholder__/` for... reasons of making the path long.

mesonbuild/backend/backends.py

@@ -781,9 +781,17 @@ class Backend:
 
     @staticmethod
     def canonicalize_filename(fname: str) -> str:
+        parts = Path(fname).parts
+        hashed = ''
+        if len(parts) > 5:
+            temp = '/'.join(parts[-5:])
+            # is it shorter to hash the beginning of the path?
+            if len(fname) > len(temp) + 41:
+                hashed = hashlib.sha1(fname.encode('utf-8')).hexdigest() + '_'
+                fname = temp
         for ch in ('/', '\\', ':'):
             fname = fname.replace(ch, '_')
-        return fname
+        return hashed + fname
 
     def object_filename_from_source(self, target: build.BuildTarget, source: 'FileOrString') -> str:
         assert isinstance(source, mesonlib.File)