Trac #30888: Make $SAGE_LOCAL/bin/sage work again from any directory, in an environment without SAGE_* variables, following symlinks

The feature introduced in #25486 (Discover `SAGE_SCRIPTS_DIR` to make
`$SAGE_LOCAL/bin/sage` work from any directory, in an environment
without SAGE_* variables) was killed by #30128 (enforce sourcing of
`sage-env-config` before `src/bin/sage-env`).

We repair it, making sure that it works even if $0 is a symlink.

URL: https://trac.sagemath.org/30888
Reported by: mkoeppe
Ticket author(s): Michael Orlitzky
Reviewer(s): Matthias Koeppe

Author: Release Manager

src/bin/sage

@@ -1,11 +1,91 @@
 #!/usr/bin/env bash
 
+# WARNING: this function is copy/pasted from both src/bin/sage-env and
+# the top-level "sage" script. Please keep them synchronized.
+resolvelinks() {
+    # $in is what still needs to be converted (normally has no starting slash)
+    in="$1"
+    # $out is the part which is converted (normally ends with trailing slash)
+    out="./"
+
+    # Move stuff from $in to $out
+    while [ -n "$in" ]; do
+        # Normalize $in by replacing consecutive slashes by one slash
+        in=$(echo "${in}" | sed 's://*:/:g')
+
+        # If $in starts with a slash, remove it and set $out to the root
+        in_without_slash=${in#/}
+        if [ "$in" != "$in_without_slash" ]; then
+            in=$in_without_slash
+            out="/"
+            continue
+        fi
+
+        # Check that the directory $out exists by trying to cd to it.
+        # If this fails, then cd will show an error message (unlike
+        # test -d "$out"), so no need to be more verbose.
+        ( cd "$out" ) || return $?
+
+
+        # Get the first component of $in
+        f=${in%%/*}
+
+        # If it is not a symbolic link, simply move it to $out
+        if [ ! -L "$out$f" ]; then
+            in=${in#"$f"}
+            out="$out$f"
+
+            # If the new $in starts with a slash, move it to $out
+            in_without_slash=${in#/}
+            if [ "$in" != "$in_without_slash" ]; then
+                in=$in_without_slash
+                out="$out/"
+            fi
+            continue
+        fi
+
+        # Now resolve the symbolic link "$f"
+        f_resolved=`readlink -n "$out$f" 2>/dev/null`
+        status=$?
+        # status 127 means readlink could not be found.
+        if [ $status -eq 127 ]; then
+            # We don't have "readlink", try a stupid "ls" hack instead.
+            # This will fail if we have filenames like "a -> b".
+            fls=`ls -l "$out$f" 2>/dev/null`
+            status=$?
+            f_resolved=${fls##*-> }
+
+            # If $fls equals $f_resolved, then certainly
+            # something is wrong
+            if [ $status -eq 0 -a "$fls" = "$f_resolved" ]; then
+                echo >&2 "Cannot parse output from ls -l '$out$f'"
+                return 1
+            fi
+        fi
+        if [ $status -ne 0 ]; then
+            echo >&2 "Cannot read symbolic link '$out$f'"
+            return $status
+        fi
+
+        # In $in, replace $f by $f_resolved (leave $out alone)
+        in="${in#${f}}"
+        in="${f_resolved}${in}"
+    done
+
+    # Return $out
+    echo "$out"
+}
+
+# Resolve the links in $0 so that local/bin/sage can be executed from
+# a symlink (Trac #30888).
+SELF=$(resolvelinks "${0}")
+
 # Display the current version of Sage
 # usage: sage_version [-v]
 #   -v    display the full version banner including release date
 sage_version() {
-    if [ -f "$0-version.sh" ]; then
-        . "$0-version.sh"
+    if [ -f "${SELF}-version.sh" ]; then
+        . "${SELF}-version.sh"
     else
         . "$SAGE_ROOT/src/bin/sage-version.sh"
     fi
@@ -60,22 +140,22 @@ usage() {
 
 # Determine SAGE_ROOT, SAGE_LOCAL, and SAGE_VENV.
 unset SAGE_VENV
-if [ -x "$0-config" ]; then
+if [ -x "${SELF}-config" ]; then
     # optional sage-config console script, installed by sage_conf
-    export SAGE_ROOT=$("$0-config" SAGE_ROOT)
-    export SAGE_LOCAL=$("$0-config" SAGE_LOCAL)
+    export SAGE_ROOT=$("${SELF}-config" SAGE_ROOT)
+    export SAGE_LOCAL=$("${SELF}-config" SAGE_LOCAL)
 fi
-if [ -f "$0-src-env-config" ]; then
+if [ -f "${SELF}-src-env-config" ]; then
     # Not installed script, present only in src/bin/
-    . "$0-src-env-config" >&2
+    . "${SELF}-src-env-config" >&2
 fi
-if [ -z "$SAGE_VENV" -a -x "$0-venv-config" ]; then
+if [ -z "$SAGE_VENV" -a -x "${SELF}-venv-config" ]; then
     # installed by setup.py
-    export SAGE_VENV=$("$0-venv-config" SAGE_VENV)
+    export SAGE_VENV=$("${SELF}-venv-config" SAGE_VENV)
 fi
-if [ -f "$0-env-config" ]; then
+if [ -f "${SELF}-env-config" ]; then
     # As of Trac #22731, sage-env-config is optional.
-    . "$0-env-config" >&2
+    . "${SELF}-env-config" >&2
 fi
 
 #####################################################################
@@ -88,7 +168,7 @@ fi
 if [ "$1" = '--nodotsage' ]; then
     export DOT_SAGE=`mktemp -d ${TMPDIR:-/tmp}/dotsageXXXXXX`
     shift
-    command "$0" "$@"
+    command "${SELF}" "$@"
     status=$?
     rm -rf "$DOT_SAGE"
     exit $status
@@ -204,14 +284,14 @@ fi
 
 
 #####################################################################
-# Source sage-env ($0 is the name of this "sage" script, so we can just
+# Source sage-env ($SELF is the name of this "sage" script, so we can just
 # append -env to that). We redirect stdout to stderr, which is safer
 # for scripts.
 #####################################################################
-if [ -f "$0-env" ]; then
-    . "$0-env" >&2
+if [ -f "${SELF}-env" ]; then
+    . "${SELF}-env" >&2
     if [ $? -ne 0 ]; then
-        echo >&2 "Error setting environment variables by sourcing '$0-env';"
+        echo >&2 "Error setting environment variables by sourcing '${SELF}-env';"
         echo >&2 "possibly contact sage-devel (see http://groups.google.com/group/sage-devel)."
         exit 1
     fi
@@ -1011,13 +1091,13 @@ if [ "$1" = '-gdb' -o "$1" = "--gdb" ]; then
     shift
     sage_setup
     if [ "$SAGE_DEBUG" = "no" ]; then
-        gdb -x "$0-gdb-commands" \
-            -args python "$0-ipython" "$@" -i
+        gdb -x "${SELF}-gdb-commands" \
+            -args python "${SELF}-ipython" "$@" -i
     else
         sage_dir=$(sage-python -c 'import os, sage; print(os.path.dirname(sage.__file__))')
         cygdb "$sage_dir" "$SAGE_SRC/sage" \
-            -- -x "$0-gdb-commands" \
-            -args python "$0-ipython" "$@" -i
+            -- -x "${SELF}-gdb-commands" \
+            -args python "${SELF}-ipython" "$@" -i
     fi
     exit $?
 fi