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Index: os-prober/common.sh
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===================================================================
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--- os-prober.orig/common.sh
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+++ os-prober/common.sh
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@@ -62,10 +62,14 @@ cache_progname() {
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   esac
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 }
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-log() {
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-  cache_progname
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-  logger -t "$progname" "$@"
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-}
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+# fd_logger: bind value now, possibly after assigning default. 
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+eval '
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+  log() {
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+    cache_progname
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+    echo "$progname: $@"  1>&'${fd_logger:=9}'
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+  }
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+'
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+export fd_logger  # so subshells inherit current value by default
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 error() {
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   log "error: $@"
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@@ -81,10 +85,14 @@ debug() {
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   fi
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 }
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-result () {
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-  log "result:" "$@"
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-  echo "$@"
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-}
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+# fd_result: bind value now, possibly after assigning default.
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+eval '
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+  result() {
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+    log "result:" "$@"
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+    echo "$@"  1>&'${fd_result:=1}'
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+  }
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+'
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+export fd_result  # so subshells inherit current value by default
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 # shim to make it easier to use os-prober outside d-i
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 if ! type mapdevfs >/dev/null 2>&1; then
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Index: os-prober/linux-boot-prober
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===================================================================
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--- os-prober.orig/linux-boot-prober
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+++ os-prober/linux-boot-prober
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@@ -1,4 +1,12 @@
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 #!/bin/sh
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+
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+# dash shell does not have "{varname}>&1" feature that bash shell has
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+# for auto-assignment of new filedescriptors.
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+# It is cumbersome to write the 'eval' to use our own variables in redirections.
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+# Therefore use fixed numbers.
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+export fd_result=3  # file descriptor for external results
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+export fd_logger=9  # file descriptor for input to logger
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+
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 . /usr/share/os-prober/common.sh
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 set -e
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@@ -19,6 +27,7 @@ bootuuid=
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 grep "^/dev/" /proc/mounts | parse_proc_mounts >"$OS_PROBER_TMP/mounted-map" || true
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+( (
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 if [ -z "$1" ]; then
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 	ERR=y
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 elif [ "$1" = btrfs -a -z "$2" ]; then
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@@ -186,3 +195,5 @@ else
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 		fi
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 	fi
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 fi
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+) 9>&1 | logger 1>&-  # fd_logger
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+) 3>&1  # fd_result
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Index: os-prober/os-prober
a27a5b
===================================================================
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--- os-prober.orig/os-prober
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+++ os-prober/os-prober
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@@ -1,7 +1,14 @@
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 #!/bin/sh
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 set -e
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-. /usr/share/os-prober/common.sh
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+# dash shell does not have "{varname}>&1" feature that bash shell has
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+# for auto-assignment of new filedescriptors.
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+# It is cumbersome to write the 'eval' to use our own variables in redirections.
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+# Therefore use fixed numbers.
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+export fd_result=3  # file descriptor for external results
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+export fd_logger=9  # file descriptor for input to logger
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+
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+ . /usr/share/os-prober/common.sh
a27a5b
 
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 newns "$@"
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 require_tmpdir
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@@ -136,6 +143,7 @@ fi
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 : >"$OS_PROBER_TMP/btrfs-vols"
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a27a5b
+( (
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 for partition in $(partitions); do
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 	if ! mapped="$(mapdevfs "$partition")"; then
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 		log "Device '$partition' does not exist; skipping"
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@@ -200,3 +208,5 @@ for partition in $(partitions); do
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 		fi
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 	fi
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 done
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+) 9>&1 | logger 1>&-  # fd_logger
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+) 3>&1  # fd_result