Automatically find externals (#15158)

Add a `spack external find` command that tries to populate
`packages.yaml` with external packages from the user's `$PATH`. This
focuses on finding build dependencies. Currently, support has only been
added for `cmake`.

For a package to be discoverable with `spack external find`, it must define:
  * an `executables` class attribute containing a list of
    regular expressions that match executable names.
  * a `determine_spec_details(prefix, specs_in_prefix)` method

Spack will call `determine_spec_details()` once for each prefix where
executables are found, passing in the path to the prefix and the path to
all found executables. The package is responsible for invoking the
executables and figuring out what type of installation(s) are in the
prefix, and returning one or more specs (each with version, variants or
whatever else the user decides to include in the spec).

The found specs and prefixes will be added to the user's `packages.yaml`
file. Providing the `--not-buildable` option will mark all generated
entries in `packages.yaml` as `buildable: False`

var/spack/repos/builtin.mock/packages/find-externals1/package.py

@@ -0,0 +1,34 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+import os
+import re
+
+
+class FindExternals1(AutotoolsPackage):
+    executables = ['find-externals1-exe']
+
+    url = "http://www.example.com/find-externals-1.0.tar.gz"
+
+    version('1.0', 'hash-1.0')
+
+    @classmethod
+    def determine_spec_details(cls, prefix, exes_in_prefix):
+        exe_to_path = dict(
+            (os.path.basename(p), p) for p in exes_in_prefix
+        )
+        if 'find-externals1-exe' not in exe_to_path:
+            return None
+
+        exe = spack.util.executable.Executable(
+            exe_to_path['find-externals1-exe'])
+        output = exe('--version', output=str)
+        if output:
+            match = re.search(r'find-externals1.*version\s+(\S+)', output)
+            if match:
+                version_str = match.group(1)
+                return Spec('find-externals1@{0}'.format(version_str))

var/spack/repos/builtin/packages/automake/package.py

@@ -5,6 +5,9 @@
 
 from spack import *
 
+import os
+import re
+
 
 class Automake(AutotoolsPackage, GNUMirrorPackage):
     """Automake -- make file builder part of autotools"""
@@ -25,6 +28,24 @@ class Automake(AutotoolsPackage, GNUMirrorPackage):
 
     build_directory = 'spack-build'
 
+    executables = ['automake']
+
+    @classmethod
+    def determine_spec_details(cls, prefix, exes_in_prefix):
+        exe_to_path = dict(
+            (os.path.basename(p), p) for p in exes_in_prefix
+        )
+        if 'automake' not in exe_to_path:
+            return None
+
+        exe = spack.util.executable.Executable(exe_to_path['automake'])
+        output = exe('--version', output=str)
+        if output:
+            match = re.search(r'GNU automake\)\s+(\S+)', output)
+            if match:
+                version_str = match.group(1)
+                return Spec('automake@{0}'.format(version_str))
+
     def patch(self):
         # The full perl shebang might be too long
         files_to_be_patched_fmt = 'bin/{0}.in'

var/spack/repos/builtin/packages/cmake/package.py

@@ -5,6 +5,9 @@
 
 from spack import *
 
+import re
+import os
+
 
 class Cmake(Package):
     """A cross-platform, open-source build system. CMake is a family of
@@ -13,6 +16,8 @@ class Cmake(Package):
     url      = 'https://github.com/Kitware/CMake/releases/download/v3.15.5/cmake-3.15.5.tar.gz'
     maintainers = ['chuckatkins']
 
+    executables = ['cmake']
+
     version('3.17.1',   sha256='3aa9114485da39cbd9665a0bfe986894a282d5f0882b1dea960a739496620727')
     version('3.17.0',   sha256='b74c05b55115eacc4fa2b77a814981dbda05cdc95a53e279fe16b7b272f00847')
     version('3.16.5',   sha256='5f760b50b8ecc9c0c37135fae5fbf00a2fef617059aa9d61c1bb91653e5a8bfc')
@@ -146,6 +151,22 @@ class Cmake(Package):
 
     phases = ['bootstrap', 'build', 'install']
 
+    @classmethod
+    def determine_spec_details(cls, prefix, exes_in_prefix):
+        exe_to_path = dict(
+            (os.path.basename(p), p) for p in exes_in_prefix
+        )
+        if 'cmake' not in exe_to_path:
+            return None
+
+        cmake = spack.util.executable.Executable(exe_to_path['cmake'])
+        output = cmake('--version', output=str)
+        if output:
+            match = re.search(r'cmake.*version\s+(\S+)', output)
+            if match:
+                version_str = match.group(1)
+                return Spec('cmake@{0}'.format(version_str))
+
     def flag_handler(self, name, flags):
         if name == 'cxxflags' and self.compiler.name == 'fj':
             cxx11plus_flags = (self.compiler.cxx11_flag,