build: say so when apt-get update fails instead of passing over it

34cb6e6 added `apt-get update;` before the install. The semicolon means
a failed update does not stop the install, which is the behaviour we
want -- one unreachable repository should not block an install the
lists already on disk can satisfy -- but it was punctuation rather than
a decision, and it was silent. The install then failed several seconds
later with "unable to locate package" and nothing tied the two
together.

Kept it non-fatal, made it explicit with `|| echo`, and wrote the
reasoning down next to it. The install itself is still the last command
in the branch, so a failed install is still the target's status.

Measured all four combinations with apt-get shimmed: a failed update
now warns and carries on, a failed install still aborts.

1 files changed, 8 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index c5cf31c..ace07d8 100644
--- a/Makefile
+++ b/Makefile
@@ -115,9 +115,16 @@ deps-linux: deps-apt deps-secp256k1 deps-randomx
 # This tree only includes two boost headers and links no boost library. The
 # apt-get update comes first: a fresh image's package lists are empty, and
 # install then fails on the first name it cannot find.
+#
+# A failed update is deliberately not fatal: one unreachable repository should
+# not block an install the lists already on disk can satisfy. It does say so,
+# though -- "unable to locate package" several seconds later is otherwise hard
+# to connect back to it. The install is the last command in the branch, so its
+# status is the target's, and that one does abort.
 deps-apt:
         @if command -v apt-get >/dev/null 2>&1; then \
-          $(SUDO) apt-get update; \
+          $(SUDO) apt-get update || \
+            echo "  warning: apt-get update failed, using the package lists on disk"; \
           $(SUDO) apt-get install -y build-essential cmake git pkg-config autoconf \
             libtool libssl-dev libdb5.3++-dev libsodium-dev nlohmann-json3-dev \
             libboost-dev libglfw3-dev libgl-dev python3-pil clang; \