download_and_compile.sh: prompt when building 'next' and logfile not already present

This ensures that users building 'next' have seen the message that warns
about possible instability and are aware of the other options (--lts for
the latest Long Term Support release, -s for the latest release).

The default answer is to continue. If --non-interactive has been given,
the prompt is skipped and download_and_compile.sh proceeds as usual.

This commit also adds _yes_no_prompt() which makes more sense than
_yes_no_quit_prompt() here, since the 'no' answer implies that the user
wants to quit.

download_and_compile.sh

@@ -97,6 +97,54 @@ function _logOutput(){
   esac
 }
 
+# Return code is 0 for 'yes' and 1 for 'no'.
+function _yes_no_prompt(){
+  local prompt="$1"
+  local default="$2"
+  local choices res answer
+
+  case "$default" in
+    [yY]) choices='Y/n' ;;
+    [nN]) choices='y/N' ;;
+    "")
+      if [[ "$INTERACTIVE_MODE" -eq 0 ]]; then
+        _printLog "Non-interactive mode requested, but found a question with" \
+                  "no default answer;"
+        _printLog "this can't work, aborting."
+        exit 1
+      fi
+      choices='y/n'
+      ;;
+    *)
+      _printLog \
+        "Invalid default choice for _yes_no_prompt(): this is a bug in the"
+        "script, aborting."
+      exit 1
+      ;;
+  esac
+
+  while true; do
+    if [[ "$INTERACTIVE_MODE" -eq 0 ]]; then
+      answer="$default"
+    else
+      read -r -p "$prompt [$choices] " answer
+    fi
+
+    if [[ -z "$answer" ]]; then
+      answer="$default"
+    fi
+
+    case "$answer" in
+      [yY]) res=0; break ;;
+      [nN]) res=1; break ;;
+      *) ;;
+    esac
+  done
+
+  return $res
+}
+
+# Return code is 0 for 'yes', 1 for 'no' and 2 for 'quit'.
 function _yes_no_quit_prompt(){
   local prompt="$1"
   local default="$2"
@@ -538,6 +586,11 @@ CBD="$PWD"
 LOGFILE="$CBD/compilation_log.txt"
 INTERACTIVE_MODE=1
 
+declare -i logfile_was_already_present_when_starting=0
+if [[ -f "$LOGFILE" ]]; then
+  logfile_was_already_present_when_starting=1
+fi
+
 # Available values for WHATTOBUILD and WHATTOBUILDALL:
 declare -a WHATTOBUILD_AVAIL=(
   'CMAKE' 'PLIB' 'OPENRTI' 'OSG' 'SIMGEAR' 'FGFS' 'DATA' 'FGRUN' 'FGO' 'FGX'
@@ -901,6 +954,21 @@ and DATA components, but other components may be affected as well. Use
 '--component-branch COMPONENT=BRANCH' (without the quotes) if you want to
 override the defaults (i.e., manually choose the branches for particular
 components)."
+
+# Make sure users building 'next' are aware of the possible consequences. :-)
+if [[ "$SELECTED_SUITE" = "next" && \
+      $logfile_was_already_present_when_starting -eq 0 ]]; then
+  set +e
+  _printLog
+  _yes_no_prompt "Are you sure you want to continue?" y; prompt_res=$?
+  set -e
+  if [[ $prompt_res -eq 1 ]]; then
+    _printLog "Aborting as requested."
+    exit 0
+  fi
+  unset -v prompt_res
+fi
+
 _printLog
 _printLog "Branch used for each component:"
 _printLog