[Robustness] nvm_alias: explicitly succeed when the alias file is unreadable

The sed/awk pipeline exits with awk's status,
so an existing-but-unreadable alias file produces empty output with status 0 - but only by accident,
alongside sed's read error on stderr.
Make that contract explicit:
a nonzero status here would flip `nvm_ensure_default_set` from "default is already set" to recreating it,
silently overwriting a write-only default alias.

nvm.sh

@@ -1362,6 +1362,14 @@ nvm_alias() {
     return 2
   fi
 
+  if [ ! -r "${NVM_ALIAS_PATH}" ]; then
+    # an existing-but-unreadable alias file yields empty output with a success
+    # status - a nonzero status here would make `nvm_ensure_default_set`
+    # overwrite an existing default alias it merely could not read
+    nvm_err "Alias file is not readable: ${NVM_ALIAS_PATH}"
+    return 0
+  fi
+
   command sed 's/#.*//; s/[[:space:]]*$//' "${NVM_ALIAS_PATH}" | command awk 'NF'
 }
 

test/fast/Aliases/nvm_alias handles unreadable alias file

@@ -0,0 +1,38 @@
+#!/bin/sh
+
+die () { echo "$@" ; cleanup ; exit 1; }
+
+cleanup() {
+  chmod 644 ../../../alias/test-edge-unreadable 2>/dev/null
+  rm -f ../../../alias/test-edge-unreadable
+}
+
+export NVM_DIR="$(cd ../../.. && pwd)"
+
+: nvm.sh
+\. "${NVM_DIR}/nvm.sh"
+
+# Create an alias file that exists but can not be read
+echo 'v0.0.1' > ../../../alias/test-edge-unreadable
+chmod 000 ../../../alias/test-edge-unreadable
+
+# root (and some containers) can read mode-000 files; nothing to assert there
+if [ -r ../../../alias/test-edge-unreadable ]; then
+  cleanup
+  exit 0
+fi
+
+# like the sed/awk pipeline this replaced: empty output, success status, so
+# that `nvm_ensure_default_set` does not overwrite an unreadable default alias
+OUTPUT="$(nvm_alias test-edge-unreadable 2>/dev/null)"
+EXIT_CODE=$?
+[ "${EXIT_CODE}" = '0' ] || die "expected exit code 0 for unreadable alias file, got ${EXIT_CODE}"
+[ -z "${OUTPUT}" ] || die "expected empty output for unreadable alias file, got >${OUTPUT}<"
+
+STDERR="$(nvm_alias test-edge-unreadable 2>&1 >/dev/null)"
+case "${STDERR}" in
+  *'not readable'*) ;;
+  *) die "expected a not-readable warning on stderr, got >${STDERR}<" ;;
+esac
+
+cleanup