build: Generate a .def file for Visual Studio-like builds

This way, we can export all the non-static symbols from the appstream DLL that
we are trying to build, without touching the headers too much, by using the
dumpbin tool that comes with every installation of Visual Studio, which clang-cl
uses.

Sadly, this approach does not allow us to effectively filter out those we mark
with the GCC hidden visibility atribute, so this is the best we can do.

src/gen-def.py

@@ -0,0 +1,116 @@
+#!/usr/bin/python3
+# pylint: disable=invalid-name,missing-docstring
+#
+# Copyright (C) 2023 Chun-wei Fan <[email protected]>
+#
+# SPDX-License-Identifier: LGPL-2.1+
+
+# Script to generate .def file from object files/static library
+
+import argparse
+import os
+import re
+import subprocess
+import sys
+
+from io import StringIO
+
+
+def run_dumpbin(objs):
+    dumpbin_results = []
+    for o in objs:
+        command = ['dumpbin', '/symbols', o]
+        p = subprocess.Popen(
+            command,
+            stdout=subprocess.PIPE,
+            stderr=None,
+            stdin=subprocess.PIPE,
+            universal_newlines=True,
+        )
+        stdout, blah = p.communicate()
+        if p.returncode != 0:
+            sys.exit(p.returncode)
+        dumpbin_results.append(stdout)
+
+    return dumpbin_results
+
+'''
+Notice output from dumpbin /symbols is like the following:
+
+Microsoft (R) COFF/PE Dumper Version 14.29.30152.0
+Copyright (C) Microsoft Corporation.  All rights reserved.
+
+
+Dump of file src\libappstream-5-core.a
+
+File Type: LIBRARY
+
+COFF SYMBOL TABLE
+000 00000000 SECT1  notype       Static       | .text
+    Section length    0, #relocs    0, #linenums    0, checksum        0
+002 00000000 SECT2  notype       Static       | .data
+    Section length    0, #relocs    0, #linenums    0, checksum        0
+004 00000000 SECT3  notype       Static       | .bss
+    Section length   18, #relocs    0, #linenums    0, checksum        0
+006 00000000 SECT4  notype       Static       | .text
+    Section length   36, #relocs    6, #linenums    0, checksum 11750A74, selection    1 (pick no duplicates)
+008 00000000 SECT4  notype ()    External     | as_video_get_type
+009 00000000 SECT32 notype       Static       | .xdata
+...
+
+So, we are only more interested in items like:
+
+008 00000000 SECT4  notype ()    External     | as_video_get_type
+
+and we need to note that these can also involve symbols that we use from
+other libraries, so we throw these out when we go through the output
+'''
+
+def extract_symbols(namespace, tokens):
+    if tokens[4] == '()':
+        check_idx = 5
+        target_idx = 7
+    else:
+        check_idx = 4
+        target_idx = 6
+
+    if tokens[check_idx] == 'External':
+        if re.match(r'^' + namespace + '_', tokens[target_idx]) is not None:
+            return tokens[target_idx]
+
+def process_dumpbin_outputs(namespace, dumpbin_results):
+    extracted_symbols = []
+    for res in dumpbin_results:
+        dumpbin_lines = StringIO(res).readlines()
+        for l in dumpbin_lines:
+            tokens = l.split()
+            if (len(tokens) == 7 or len(tokens) == 8) and tokens[3] == 'notype':
+                result = extract_symbols(namespace, tokens)
+                if result is not None:
+                    extracted_symbols.append(result)
+    extracted_symbols.sort()
+    return extracted_symbols
+
+
+def output_def_file(def_file, symbols):
+    with open(def_file, "w") as out:
+        out.write("EXPORTS\n")
+        for s in symbols:
+            out.write("%s\n" % s)
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description='Generate .def from object files or static libs.')
+    parser.add_argument('-n', '--namespace', dest='namespace',
+                        help='namespace of symbols', required=True)
+    parser.add_argument('--def', dest='def_file', required=True,
+                        help='output def file')
+    parser.add_argument('objects', nargs='+',
+                        help='objects files/static libraries to process')
+
+    args = parser.parse_args()
+    namespace = args.namespace
+    def_file = args.def_file
+    objs = args.objects
+    dumpbin_results = run_dumpbin(objs)
+    output_def_file(def_file, process_dumpbin_outputs(namespace, dumpbin_results))

src/meson.build

@@ -219,11 +219,27 @@ as_deps = [
 
 as_libs = [as_core_lib]
 
+if is_windows and cc.get_argument_syntax() == 'msvc'
+  gen_def = find_program('gen-def.py')
+  as_def = custom_target(
+    'Generate Windows .def file',
+	input: as_core_lib,
+	output: 'appstream.def',
+    command: [
+        gen_def,
+        '-n', 'as',
+        '--def', '@OUTPUT@',
+        '@INPUT@',
+    ]
+  )
+endif
+
 appstream_lib = library ('appstream',
     link_whole: as_libs,
     soversion: as_api_level,
     version: as_version,
     dependencies: as_deps,
+    vs_module_defs: as_def,
     install: true
 )