Add validation for duplicate xml elements

[oturpe]

Draft fix of #401

As an example, duplicate checking for two elements is added. Using this simple system, checking all elements requires replicating the code for each checked field. Solution with less replication would require a new data structure and passing it around: instead of enumerated problems like AS_APP_PROBLEM_DUPLICATE_PROJECT_LICENSE, some kind of generic duplicate problem entry that takes element name as input.

Posting this draft for review, because I am really new to this codebase and cannot say what would be best way to proceed:

1. Replicate the simple solution for all fields that can only appear once
2. Develop the generic solution
3. Only check project_license duplicates, as that is what was reported in Validation should not accept multiple project_license elements #401 and there the root of the problem is that even if a project may be under multiple licenses, there can only be one license field and separate syntax for encoding everything in there must be used.

Comments welcome.

[hughsie]

A few nits, but looks basically what I would have done, so good job.

[oturpe]

Formatting fixed. A test case failed, apparently the "bad file" test already had a test file that had these problems, now the asserts need to be fixed. I will see to that soon.

[oturpe]

Tests added. In absence of further comments about which elements should be tested for duplicates, I left the scope as it was initially: only the license fields.

[oturpe]

Travis CI build failed, but to me that looks like an unrelated issue, as if something is generated without strict ordering, but is checked against fixed expectation:

Bail out! As:ERROR:../libappstream-builder/asb-self-test.c:524:asb_test_context_func: assertion failed (error == NULL):
--- /tmp/b	2021-05-17 19:42:30.984710347 +0000
+++ /tmp/a	2021-05-17 19:42:30.984710347 +0000
@@ -24,8 +24,8 @@
 <name>App</name>
 <summary>A test application</summary>
 <description><p>Long description goes here.</p></description>
-<icon type="cached" height="128" width="128">app.png</icon>
 <icon type="cached" height="64" width="64">app.png</icon>
+<icon type="cached" height="128" width="128">app.png</icon>
 <categories>
 <category>Profiling</category>
 <category>System</category>
 (-g-type-private--GTypeFlags, 0)

[hughsie]

but to me that looks like an unrelated issue

Agreed! Could I be cheeky and ask you to add the fix for that to this branch please? It should just be reordering the expected lines in asb-self-test.c. Thanks!

[oturpe]

That error is now fixed, but another one was hidden behind it:

Bail out! As:ERROR:../libappstream-builder/asb-self-test.c:628:asb_test_context_func: assertion failed: (g_file_test ("/tmp/asbuilder/temp/icons/64x64/app.png", G_FILE_TEST_EXISTS))

--- stderr ---
**
As:ERROR:../libappstream-builder/asb-self-test.c:628:asb_test_context_func: assertion failed: (g_file_test ("/tmp/asbuilder/temp/icons/64x64/app.png", G_FILE_TEST_EXISTS))
-------

[hughsie]

That error is now fixed, but another one was hidden behind it:

Okay, I can look at that later. Thanks for the fixes!