Looking more around, '\n' are added to several values:

See https://commons.wikimedia.org/wiki/Special:ApiSandbox#action=query&prop=imageinfo&format=json&iiprop=extmetadata&iilimit=10&titles=File%3ACompans%20lake%20-%20Anas%20platyrhynchos%2007.JPG :

"Credit": {
"value": "\nSelf-photographed",
"source": "commons-desc-page",
"hidden": ""
},
"LicenseUrl": {
"value": "http://creativecommons.org/licenses/by-sa/3.0\n",
"source": "commons-desc-page",
"hidden": ""
},
"LicenseShortName": {
"value": "CC-BY-SA-3.0\n",
"source": "commons-desc-page",
"hidden": ""
},
"UsageTerms": {
"value": "Creative Commons Attribution-Share Alike 3.0\n",
"source": "commons-desc-page",
"hidden": ""
},

Change 97743 had a related patch set uploaded by Gergő Tisza:
Trim HTML-based metadata values

https://gerrit.wikimedia.org/r/97743

Change 97743 abandoned by Gergő Tisza:
Trim HTML-based metadata values

Reason:
Abandoning this change since InformationParser has been completely rewritten in the meantime.

https://gerrit.wikimedia.org/r/97743

Change 120948 had a related patch set uploaded by Gergő Tisza:
Clean parsed HTML

https://gerrit.wikimedia.org/r/120948

Change 120948 merged by jenkins-bot:
Clean parsed HTML

https://gerrit.wikimedia.org/r/120948

This issue is occurring again. See e.g. https://commons.wikimedia.org/w/api.php?action=query&prop=imageinfo&format=json&iiprop=commonmetadata|extmetadata&iilimit=1&titles=File%3ALandsort%20Lighthouse%20August%202013%2009.jpg

where
"LicenseShortName": {

"value": "CC-BY-SA-3.0\n",
"source": "commons-desc-page",
"hidden": ""

},
"UsageTerms": {

"value": "Creative Commons Attribution-Share Alike 3.0\n",
"source": "commons-desc-page",
"hidden": ""

},
"LicenseUrl": {

"value": "http://creativecommons.org/licenses/by-sa/3.0\n",
"source": "commons-desc-page",
"hidden": ""

},

Looking at the html source of the example above [1] there is no trace of these newline characters. Hence it might not be a cleaning/trimming issue in the TemplateParser but rather inserted by it?

[1] https://commons.wikimedia.org/wiki/File:Landsort_Lighthouse_August_2013_09.jpg

• Bug 69497 has been marked as a duplicate of this bug. ***

As stated in bug 69497, these newlines are in the license template, and the code doing the HTML scraping there had better remove them.

The code to remove is in https://gerrit.wikimedia.org/r/#/c/120948/1/TemplateParser.php which at a glance seems correct to me. Also, Lokal_Profil is right that the newline is not always present in the HTML code. I'll test locally with the examples mentioned here.

This code does _not_ look good. '/^\s+(.*)\s+$/' is wrong. It fails to trim if there are no leading blanks (or no trailing blanks). And watch out for the greedy (.*), that also looks wrong.

(In reply to Tisza Gergő from comment #10)

Also, Lokal_Profil is right that the newline is
not always present in the HTML code. I'll test locally with the examples
mentioned here.

Not correct. See

https://commons.wikimedia.org/w/api.php?action=query&prop=imageinfo&iiprop=extmetadata&format=jsonfm&titles=File:Landsort_Lighthouse_August_2013_09.jpg

Returns the same trailing newlines for UsageTerms and LicenseUrl.

Change 155901 had a related patch set uploaded by TheDJ:
TemplateParser: Fix whitespace trim

https://gerrit.wikimedia.org/r/155901

Change 155901 merged by jenkins-bot:
TemplateParser: Fix whitespace trim

https://gerrit.wikimedia.org/r/155901

(In reply to Lupo from comment #11)

This code does _not_ look good. '/^\s+(.*)\s+$/' is wrong. It fails to trim
if there are no leading blanks (or no trailing blanks). And watch out for
the greedy (.*), that also looks wrong.

D'oh, that was stupid. Thanks for fixing, Lupo & TheDJ!

• Bug 66652 has been marked as a duplicate of this bug. ***