e2e 744 depended on the host having a C runtime, and on a key mcpp ignores

TWO DEFECTS IN ONE TEST, AND THE SHARD FOUND THE FIRST.

The link it examines was an ordinary hosted one, so it needed startup files
that the graph's C library --- a marker package supplying no C library at all
--- does not provide. They came from the host: present on a developer's
machine, absent in the container the Linux shard runs in, where the link died
before it ever reached the symbol under examination:

    /usr/bin/ld: cannot find crt1.o: No such file or directory

A test whose subject is a link diagnostic must not depend on anything else
about the link succeeding up to that point. It is freestanding now
(`-nostdlib -nostartfiles -static`, and an entry point of its own), so exactly
one symbol is undefined and it is the one being examined.

THE SECOND DEFECT WAS IN THE REPAIR. Those flags were first written as
`[targets.<name>] ldflags`, which is not a key mcpp has: the manifest reports
`unsupported key 'ldflags' (ignored)` and carries on. The link then still
failed, still named the symbol, and still carried the note, so all four legs
passed while the arrangement they rest on had not happened. `build.ldflags` is
the spelling that exists, and the flags are now measured on the emitted link
line rather than assumed.

Leg E asserts the arrangement: no key was ignored, and the link never reached
the host's C runtime. Putting the flags back in the target fails E and only E
--- A through D go on passing, which is the whole reason E is there. A warning
printed into a passing test is invisible.

Author: speak-agent

tests/e2e/744_a_declared_absence_explains_the_link_that_asked_for_it.sh

@@ -21,6 +21,27 @@
 # THE SYMBOL IS DELIBERATELY ONE NOTHING DEFINES. `fork` is the real row in
 # openkal-musl's manifest and is defined by every C library on a Linux host,
 # so a test written with it would link and assert nothing.
+#
+# AND THE LINK IS FREESTANDING, WHICH IS NOT A STYLISTIC CHOICE. Written as an
+# ordinary hosted program this test passed here and failed on the shard, where
+# the link died before it ever reached the symbol:
+#
+#     /usr/bin/ld: cannot find crt1.o: No such file or directory
+#
+# The graph's C library is a marker package that supplies no C library at all,
+# so the startup files came from the host --- present on a developer's machine,
+# absent in the container the shard runs in. A test whose subject is a link
+# diagnostic must not depend on anything else about the link succeeding up to
+# that point. `-nostdlib -nostartfiles -static` and an entry point of its own
+# leave exactly one undefined symbol, which is the one under examination.
+#
+# THE FLAGS GO IN `[build]`, NOT IN THE TARGET, AND LEG E ASSERTS THAT THEY
+# ARRIVED. Written as `[targets.<name>] ldflags` they are not a key mcpp has:
+# the manifest reports `unsupported key 'ldflags' (ignored)` and carries on.
+# The link then still failed, still named the symbol, and still carried the
+# note --- so all four legs below passed while the thing they were arranged
+# around had not happened. Leg E is there because a warning printed into a
+# passing test is invisible.
 set -e
 
 MCPP="${MCPP:-mcpp}"
@@ -34,7 +55,9 @@ int fake_c_library_marker(void) { return 0; }
 EOF
 cat > src/main.c <<'EOF'
 extern int mcpp_absent_probe_fn(void);
-int main(void) { return mcpp_absent_probe_fn(); }
+/* `_start`, not `main`: this links without startup files, so there is no C
+   runtime to call main. Nothing runs it -- the subject is the link. */
+void _start(void) { mcpp_absent_probe_fn(); }
 EOF
 cat > mcpp.toml <<'EOF'
 [package]
@@ -50,6 +73,7 @@ fakelibc = { path = "libc" }
 
 [build]
 allow_host_libs = true
+ldflags = ["-nostdlib", "-nostartfiles", "-static"]
 EOF
 cat > libc/mcpp.toml <<'EOF'
 [package]
@@ -107,4 +131,24 @@ echo "$out" | grep -q "not a defect in the build" || {
 }
 echo "OK: D (the note distinguishes an absence from a defect)"
 
+# ── E. the arrangement this test rests on actually happened ────────────────
+# An unsupported manifest key is a warning and not an error, so a misplaced
+# `ldflags` leaves every assertion above passing for the wrong reason: the
+# link succeeded as far as the host's startup files allowed, which is a
+# property of the machine rather than of this graph.
+echo "$out" | grep -q "unsupported key" && {
+    echo "FAIL: the manifest this test builds has a key mcpp ignored, so the" \
+         "link above was not the one this test describes" >&2
+    echo "$out" | grep "unsupported key" >&2
+    exit 1
+}
+echo "$out" | grep -qiE "crt1|crti|multiple definition" && {
+    echo "FAIL: the link reached the host's C runtime. The flags that keep it" \
+         "freestanding did not take effect, and what failed is the machine" \
+         "this ran on rather than the absence under examination." >&2
+    echo "$out" >&2
+    exit 1
+}
+echo "OK: E (the link was freestanding, as this test requires)"
+
 echo "OK"