TIP 538 prevents building Tkinter against Tcl 8.7 built with bundled libtommath

[chrstphrchvz]

Bug report

Separated out from #103194 (comment) since I believe this likely should be addressed first.

TIP 538 means that Tcl 8.7 can be built to use separately-built libtommath, and no longer recommends building with the bundled libtommath. tclTomMath.h requires tommath.h, which Tcl will no longer install a copy of even when the bundled libtommath is used (as another user recently found out: https://core.tcl-lang.org/tcl/info/ff255adc4cb5). Either tommath.h must be obtained separately, or TCL_NO_TOMMATH_H must be defined before including tclTomMath.h.

Package managers presumably would want to build Tcl against separately-built libtommath once they update to 8.7, and in turn build Tkinter against separately-built Tcl and libtommath. But given that nothing else in CPython uses libtommath, how much would the official Python binary distributions prefer that Tcl continue building its bundled libtommath?

Linked PRs

• gh-103839: Allow building Tkinter against Tcl 8.7 without external libtommath #103842

[ned-deily]

Presumably this is only an issue for the Windows builds and for macOS installer builds? My initial reaction is that we provide a Tcl binary solely to run Tk in turn solely to run tkinter-based applications; we have no need to provide a full Tcl installation with all its third-party dependencies as many of them duplicate functionality provided directly by Python and can only somewhat indirectly used via tkinter. So, if libtommath falls into that category, adding a something (like TCL_NO_TOMMATH_H to our builds of Tcl seems fine. Other opinions?

[terryjreedy]

@serhiy-storchaka @zooba tcl/tk build question. I suspect your answer will match Ned's.

[zooba]

Yeah, I'd rather omit the dependency. (I couldn't find a nice way to omit the zlib DLL dependency, which is why we now have to carry a second copy of zlib, but let's not use that as precedent to add more unnecessary weight.)

[terryjreedy]

@ned-deily @zooba Do you comments constitute approval of #103842? (I suspect so but don't understand build stuff.) If so, please say so there and maybe one of you merge it. Should it be backported?

[zooba]

That PR seems to be a fairly trivial preprocessor check? Assuming it works, it doesn't bother me at all. Python 3.13 is the first that might include Tcl 8.7 on Windows, so if it helps people who are building on other platforms, it can go wherever it likes.

Or did you mean a different PR?