Fix the semihosting file open modes for update and append - #4312

open_file in probe-rs-tools/src/bin/probe-rs/rpc/utils/semihosting.rs maps the ARM semihosting SYS_OPEN mode strings onto File::options(). Three of the six rows do not do what C fopen does, so a target that opens a file for update or for appending gets the wrong file.

Line 196, "r+" | "r+b", sets truncate(true). fopen opens r+ for update and leaves the contents alone, so a target that reopened a file to patch part of it found the file empty.

Line 199, "a" | "ab", sets write(true) and never append(true), so the writes land at offset 0 and overwrite the start of the file. Writing ab to a file holding 0123456789 produced ab23456789 instead of 0123456789ab.

Line 200, "a+" | "a+b", sets read(true).write(false).create(true). create needs write or append access, so File::options().open() returns InvalidInput with "creating or truncating a file requires write or append access" and a+ never opens a file at all. That is the same std rule #3960 ran into for r, which fixed r only.

The fix is truncate(false) for r+, and append(true) for a and a+. w and w+ were already right.

The tests call open_file directly, so they need no probe and no target. They cover the five distinct behaviours of the mode table. Against the mode table on master, r and w pass and the other three fail:

test rpc::utils::semihosting::test::read_does_not_truncate ... ok
test rpc::utils::semihosting::test::write_truncates ... ok
test rpc::utils::semihosting::test::read_update_does_not_truncate ... FAILED
test rpc::utils::semihosting::test::append_writes_to_the_end ... FAILED
test rpc::utils::semihosting::test::append_update_writes_to_the_end ... FAILED

---- read_update_does_not_truncate ----
  left: []
 right: [48, 49, 50, 51, 52, 53, 54, 55, 56, 57]

---- append_writes_to_the_end ----
  left: [97, 98, 50, 51, 52, 53, 54, 55, 56, 57]
 right: [48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 97, 98]

---- append_update_writes_to_the_end ----
mode a+ failed to open the file

With the change, on macOS 26.2 arm64 with rustc 1.95.0:

cargo test -p probe-rs-tools --bin probe-rs semihosting:: gives test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 150 filtered out.

cargo fmt --all -- --check reports no diff.

cargo clippy -p probe-rs-tools --all-features --all-targets --locked -- -D warnings -A clippy::collapsible_match finishes clean. The allow covers util/cargo.rs:86, which the clippy in my toolchain also rejects on master, before this change.

Must

• The code compiles without errors or warnings.
• All tests pass and in the best case you also added new tests.
• cargo fmt was run.
• Add a changelog fragment describing your changes in changelogs/. See https://github.com/probe-rs/probe-rs/blob/master/CONTRIBUTING.md#changelog-entries.

Nice to have

• You add a description of your work to this PR.
• You added proper docs (in code, rustdoc and README.md) for your newly added features and code.