scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
    scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
  it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

scripts/functions

@@ -6,40 +6,41 @@
 CT_OnError() {
     local ret=$?
     local intro
-    local file
-    local line
-    local func
-    local step_depth
+    local file line func
+    local step step_depth
 
-    # Bail out early in subshell, the upper level shell will act accordingly.
-    [ ${BASH_SUBSHELL} -eq 0 ] || exit $ret
+    # To avoid printing the backtace for each sub-shell
+    # up to the top-level, just remember we've dumped it
+    if [ ! -f "${CT_BUILD_DIR}/backtrace" ]; then
+        touch "${CT_BUILD_DIR}/backtrace"
 
         # Print steps backtrace
         step_depth=${CT_STEP_COUNT}
-    CT_STEP_COUNT=2
+        CT_STEP_COUNT=1     # To have a zero-indentation
         CT_DoLog ERROR ""
+        CT_DoLog ERROR ">>"
         intro="Build failed"
-    for((step=step_depth; step>1; step--)); do
+        for((step=step_depth; step>0; step--)); do
             CT_DoLog ERROR ">>  ${intro} in step '${CT_STEP_MESSAGE[${step}]}'"
             intro="      called"
         done
 
         # Print functions backtrace
         intro="Error happened in"
-    offset=1
         CT_DoLog ERROR ">>"
         for((depth=1; ${BASH_LINENO[$((${depth}-1))]}>0; depth++)); do
             file="${BASH_SOURCE[${depth}]#${CT_LIB_DIR}/}"
-        case "${depth}" in
-            1)  line="";;
-            *)  line="@${BASH_LINENO[${depth}-1]}"
-        esac
             func="${FUNCNAME[${depth}]}"
+            line="@${BASH_LINENO[${depth}-1]:-?}"
             CT_DoLog ERROR ">>  ${intro}: ${func}[${file}${line}]"
             intro="      called from"
         done
+    fi
 
+    # And finally, in top-level shell, print some hints
+    if [ ${BASH_SUBSHELL} -eq 0 ]; then
         # Help diagnose the error
+        CT_STEP_COUNT=1     # To have a zero-indentation
         CT_DoLog ERROR ">>"
         if [ "${CT_LOG_TO_FILE}" = "y" ]; then
             CT_DoLog ERROR ">>  For more info on this error, look at the file: '${tmp_log_file#${CT_TOP_DIR}/}'"
@@ -47,11 +48,10 @@ CT_OnError() {
         CT_DoLog ERROR ">>  There is a list of known issues, some with workarounds, in:"
         CT_DoLog ERROR ">>      '${CT_DOC_DIR#${CT_TOP_DIR}/}/B - Known issues.txt'"
 
-    CT_DoLog ERROR ""
-    CT_DoLog ERROR "Build failed in step '${CT_STEP_MESSAGE[${CT_STEP_COUNT}]}'"
-
         CT_DoLog ERROR ""
         CT_DoEnd ERROR
+        rm -f "${CT_BUILD_DIR}/backtrace"
+    fi
     exit $ret
 }
 
@@ -288,7 +288,7 @@ CT_DoDate() {
 }
 
 CT_STEP_COUNT=1
-CT_STEP_MESSAGE[${CT_STEP_COUNT}]="<none>"
+CT_STEP_MESSAGE[${CT_STEP_COUNT}]="(top-level)"
 # Memorise a step being done so that any error is caught
 # Usage: CT_DoStep <loglevel> <message>
 CT_DoStep() {