Failsafe checks for available APT package

When checking for available packages in APT repository, running a dry-run install can fail for other reasons, even if the package is available. Currently, in such case, wrong fallback packages are selected: https://github.com/pi-hole/pi-hole/issues/2888
"apt-cache show <pkg>" is a quicker method to check for available packages. This is now done as well to check if the fallbacks are available. If none is found, the installer exits with meaningful error message and exit code.

In rare cases, the APT list files can be missing when the installer is started. E.g. this could be on a fresh system, APT lists could have been moved to RAM or removed as a cleanup step. "apt-cache" calls will then fail, same as dry-run installs were. To assure that current package lists are checked, update the package cache directly after the Ubuntu universe repo has been added, only in the Debian/Ubuntu block. This renders the variable handling in RH/Fedora block obsolete.

Signed-off-by: MichaIng <micha@dietpi.com>

automated install/basic-install.sh

@@ -192,8 +192,8 @@ if is_command apt-get ; then
     APT_SOURCES="/etc/apt/sources.list"
     if awk 'BEGIN{a=1;b=0}/bionic main/{a=0}/bionic.*universe/{b=1}END{exit a + b}' ${APT_SOURCES}; then
         if ! whiptail --defaultno --title "Dependencies Require Update to Allowed Repositories" --yesno "Would you like to enable 'universe' repository?\\n\\nThis repository is required by the following packages:\\n\\n- dhcpcd5" "${r}" "${c}"; then
-            printf "  %b Aborting installation: dependencies could not be installed.\\n" "${CROSS}"
-            exit # exit the installer
+            printf "  %b Aborting installation: Dependencies could not be installed.\\n" "${CROSS}"
+            exit 1 # exit the installer
         else
             printf "  %b Enabling universe package repository for Ubuntu Bionic\\n" "${INFO}"
             cp -p ${APT_SOURCES} ${APT_SOURCES}.backup # Backup current repo list
@@ -202,14 +202,18 @@ if is_command apt-get ; then
             printf "  %b Enabled %s\\n" "${TICK}" "'universe' repository"
         fi
     fi
-    # Debian 7 doesn't have iproute2 so if the dry run install is successful,
-    if "${PKG_MANAGER}" install --dry-run iproute2 > /dev/null 2>&1; then
-        # we can install it
+    # Update package cache. This is required already here to assure apt-cache calls have package lists available.
+    update_package_cache || exit 1
+    # Debian 7 doesn't have iproute2 so check if it's available first
+    if apt-cache show iproute2 > /dev/null 2>&1; then
         iproute_pkg="iproute2"
-    # Otherwise,
-    else
-        # use iproute
+    # Otherwise, check if iproute is available
+    elif apt-cache show iproute > /dev/null 2>&1; then
         iproute_pkg="iproute"
+    # Else print error and exit
+    else
+        printf "  %b Aborting installation: iproute2 and iproute packages were not found in APT repository.\\n" "${CROSS}"
+        exit 1
     fi
     # Check for and determine version number (major and minor) of current php install
     if is_command php ; then
@@ -224,21 +228,28 @@ if is_command apt-get ; then
     # Check if installed php is v 7.0, or newer to determine packages to install
     if [[ "$phpInsNewer" != true ]]; then
         # Prefer the php metapackage if it's there
-        if "${PKG_MANAGER}" install --dry-run php > /dev/null 2>&1; then
+        if apt-cache show php > /dev/null 2>&1; then
             phpVer="php"
-        # fall back on the php5 packages
-        else
+        # Else fall back on the php5 package if it's there
+        elif apt-cache show php5 > /dev/null 2>&1; then
             phpVer="php5"
+        # Else print error and exit
+        else
+            printf "  %b Aborting installation: No PHP packages were found in APT repository.\\n" "${CROSS}"
+            exit 1
         fi
     else
         # Newer php is installed, its common, cgi & sqlite counterparts are deps
         phpVer="php$phpInsMajor.$phpInsMinor"
     fi
     # We also need the correct version for `php-sqlite` (which differs across distros)
-    if "${PKG_MANAGER}" install --dry-run "${phpVer}-sqlite3" > /dev/null 2>&1; then
+    if apt-cache show "${phpVer}-sqlite3" > /dev/null 2>&1; then
         phpSqlite="sqlite3"
-    else
+    elif apt-cache show "${phpVer}-sqlite" > /dev/null 2>&1; then
         phpSqlite="sqlite"
+    else
+        printf "  %b Aborting installation: No SQLite PHP module was found in APT repository.\\n" "${CROSS}"
+        exit 1
     fi
     # Since our install script is so large, we need several other programs to successfully get a machine provisioned
     # These programs are stored in an array so they can be looped through later
@@ -281,8 +292,6 @@ elif is_command rpm ; then
         PKG_MANAGER="yum"
     fi
 
-    # Fedora and family update cache on every PKG_INSTALL call, no need for a separate update.
-    UPDATE_PKG_CACHE=":"
     PKG_INSTALL=("${PKG_MANAGER}" install -y)
     PKG_COUNT="${PKG_MANAGER} check-update | egrep '(.i686|.x86|.noarch|.arm|.src)' | wc -l"
     INSTALLER_DEPS=(git iproute newt procps-ng which chkconfig)
@@ -2585,9 +2594,6 @@ main() {
         verifyFreeDiskSpace
     fi
 
-    # Update package cache
-    update_package_cache || exit 1
-
     # Notify user of package availability
     notify_package_updates_available