Document TARGET naming mechanisms in Jamulus.pro

[mcfnord]

MY LLM WROTE:

Follow-up to @pljones' review comment on #3799 (#3799 (comment)):

It might be worth documenting in the Jamulus.pro rather than surfacing it to the debian-specific README. Anyone maintaining Jamulus.pro would need to be aware of the intent.

Probably a separate PR, though, at this point.

This is that separate PR. Comment only — no build behaviour changes.

What it documents

The two Debian packages get their binary names by different mechanisms, and nothing in Jamulus.pro said so:

package	qmake invocation	resulting TARGET
jamulus	CONFIG+=noupcasename (linux/debian/rules:34)	jamulus
jamulus-headless	TARGET=jamulus-headless on the command line (linux/debian/rules:35)	jamulus-headless
(neither)	plain qmake	Jamulus

TARGET is consumed in three places, so a change here is not local:

• usr/bin/<TARGET> is what debian/jamulus.install and debian/jamulus-headless.install list;
• it is substituted into Exec= in the generated .desktop files (linux/jamulus.desktop.in:13 is Exec=$$TARGET);
• it has to keep matching the hardcoded ExecStart=/usr/bin/jamulus-headless in debian/jamulus-headless.service:21.

The part worth knowing

TARGET = jamulus inside the noupcasename block silently overrides a TARGET= passed on the qmake command line. So adding noupcasename to the headless build — which looks like a harmless tidy-up that unifies the two mechanisms — would rename that binary to jamulus, and the failure surfaces as a broken package, not a build error.

How this was checked

Each row of the table above was produced by running qmake and reading TARGET out of the generated Makefile.Release, not by reading the .pro file. The Exec= claim was checked against the generated .desktop files (Exec=jamulus with noupcasename, Exec=Jamulus without). Environment: qmake 3.1 / Qt 5.15.13, Ubuntu, QT_SELECT=qt5 as linux/debian/rules:3 pins.

Not exercised: the macOS server_bundle append at Jamulus.pro:191 (TARGET = $${TARGET}Server), and the precedence rule was confirmed on Qt5 only. Neither affects Debian packaging, but say the word if you want either measured before merge.

[pljones]

it is substituted into Exec= in the generated .desktop files (linux/jamulus.desktop.in:13 is Exec=$$TARGET);
it has to keep matching the hardcoded ExecStart=/usr/bin/jamulus-headless in debian/jamulus-headless.service:21.

I'm tempted to say both should have TARGET substituted in for consistency.

TARGET = jamulus inside the noupcasename block silently overrides a TARGET= passed on the qmake command line.

I'd say this is a bug. The AutoBuild workflow should pass in the TARGET for both, making it clear it's not the default build behaviour.

That would remove any need for noupcasename in the Jamulus.pro and AutoBuild workflows, I think - and have the added benefit of consistency.

(It would need noting as a breaking change for people cooking their own debs.)