[pankgeorg]

This removes two of the blockers to using Tar in programs compiled with juliac --trim=safe. With JuliaIO/ArgTools.jl#46 and a Base change (below), reading and extracting tarballs verifies cleanly; the change is correct and tested on its own.

Specialize on callbacks that are only passed on. The API functions and their helpers take the predicate or callback as f::Function and pass it on: to the next function, to a do block, or, for every method with keyword arguments, to the method's keyword body. Julia does not specialize a method on a Function argument it only passes on (performance tips), so these methods are compiled for f::Function and calling the callback is a dynamic dispatch. After calling create, list, extract, rewrite and tree_hash with closures, 31 method instances of Tar have Function in their signature on master; with this change, none. The callbacks are now typed f::F where {F <: Function}; the signatures, and so dispatch, are unchanged.

Take a header error's message as the String it is. read_standard_header converted the message of the ErrorException thrown by its header checks with String(err.msg), a dynamic call (the field is an AbstractString). Both checks throw through header_error, which calls error with a String, so the message is asserted to be one instead.

Effect on juliac --trim=safe (JuliaC 0.3.12, Julia master 0d77de6d2c). A program that calls Tar.list (with and without a callback) and Tar.extract with a predicate on a tarball path gets 0 verifier errors with this change, JuliaIO/ArgTools.jl#46 and JuliaLang/julia#64448, so that show of a Symbol does not compile the expression printer (check_header shows a Symbol through repr(hdr)), and 264 with only the first two. On master Tar.list(path) stops at ArgTools (2 errors in open_nolock). Adding create, tree_hash and rewrite to that program leaves 8 errors, all in create, all from repr of a Dict in write_tarball's error message, which is Base's show for Pairs and not changed here.

Cost. More methods are compiled per callback type. First calls in a fresh process, in this order (median of 5, master; this change vs. without): with the default predicate create 167 ms vs. 168, list 137 vs. 151, extract 146 vs. 165, tree_hash 107 vs. 112, rewrite 71 vs. 74; then with new user closures create(pred) 14 vs. 14 and extract(pred) 55 vs. 27, the extract path being compiled for the new closure type.

Tests. A new testset calls the API with closures in a fresh Julia process and checks that no Tar method was compiled with Function in its signature (a fresh process, since inference and reflection elsewhere can compile such instances); it fails on master, listing them. The full test suite passes on Julia 1.13.2 and on master.

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https://claude.ai/code/session_01HN2L2VxELK7cEP7rLbYHoC