feat: Implement a two-phase init which allows us to write normal init scripts (#168)

Implement a two-phase init procedure in starship. The first phase causes the shell to source a subshell, while the second phase (in the subshell) prints the main init script.

This allows us to have nice init scripts with good styling, comments, and no pile of semicolons. Even better, it works as a drop-in replacement, so we don't need to update the docs.

2 files changed, 168 insertions, 109 deletions

diff --git a/src/init.rs b/src/init.rs
index 1479d2174..4fb6882ef 100644
--- a/src/init.rs
+++ b/src/init.rs
@@ -1,26 +1,39 @@
 use std::ffi::OsStr;
 use std::path::Path;
 
-/* We need to send execution time to the prompt for the cmd_duration module. For fish,
-this is fairly straightforward. For bash and zsh, we'll need to use several
-shell utilities to get the time, as well as render the prompt */
-
-pub fn init(shell_name: &str) {
+/* We use a two-phase init here: the first phase gives a simple command to the
+shell. This command evaluates a more complicated script using `source` and
+process substitution.
+
+Directly using `eval` on a shell script causes it to be evaluated in
+a single line, which sucks because things like comments will comment out the
+rest of the script, and you have to spam semicolons everywhere. By using
+source and process substitutions, we make it possible to comment and debug
+the init scripts. */
+
+/* This prints the setup stub, the short piece of code which sets up the main
+init code. The stub produces the main init script, then evaluates it with
+`source` and process substitution */
+pub fn init_stub(shell_name: &str) {
     log::debug!("Shell name: {}", shell_name);
 
     let shell_basename = Path::new(shell_name).file_stem().and_then(OsStr::to_str);
 
-    let setup_script = match shell_basename {
+    let setup_stub = match shell_basename {
         Some("bash") => {
-            let script = BASH_INIT;
+            /* This *should* look like the zsh function, but bash 3.2 (MacOS default shell)
+            does not support using source with process substitution, so we use this
+            workaround from https://stackoverflow.com/a/32596626 */
+            let script = "source /dev/stdin <<<\"$(starship init bash --print-full-init)\"";
             Some(script)
         }
         Some("zsh") => {
-            let script = ZSH_INIT;
+            let script = "source <(starship init zsh --print-full-init)";
             Some(script)
         }
         Some("fish") => {
-            let script = FISH_INIT;
+            // Fish does process substitution with pipes and psub instead of bash syntax
+            let script = "source (starship init fish --print-full-init | psub)";
             Some(script)
         }
         None => {
@@ -46,142 +59,179 @@ pub fn init(shell_name: &str) {
             None
         }
     };
+    if let Some(script) = setup_stub {
+        print!("{}", script);
+    };
+}
 
+/* This function (called when `--print-full-init` is passed to `starship init`)
+prints out the main initialization script */
+pub fn init_main(shell_name: &str) {
+    let setup_script = match shell_name {
+        "bash" => Some(BASH_INIT),
+        "zsh" => Some(ZSH_INIT),
+        "fish" => Some(FISH_INIT),
+        _ => {
+            println!(
+                "printf \"Shell name detection failed on phase two init.\\n\
+                 This probably indicates a bug within starship: please open\\n\
+                 an issue at https://github.com/starship/starship/issues/new\\n\""
+            );
+            None
+        }
+    };
     if let Some(script) = setup_script {
         print!("{}", script);
-    }
+    };
 }
 
-/*
- For bash: we need to manually hook functions ourself: PROMPT_COMMAND will exec
-   right before the prompt is drawn, and any function trapped by DEBUG will exec
-   before a command is run.
-
-   There is a preexec/precmd framework for bash out there: if we find the
-   appropriate variables set, assume we are using that framework:
-   https://github.com/rcaloras/bash-preexec
-
-   Bash quirk: DEBUG is triggered whenever a command is executed, even if that
-   command is part of a pipeline. To avoid only timing the last part of a pipeline,
-   we only start the timer if no timer has been started since the last prompt draw,
-   tracked by the variable PREEXEC_READY. Similarly, only draw timing info if
-   STARSHIP_START_TIME is defined, in case preexec was interrupted.
-
-   Finally, to work around existing DEBUG traps in the absence of a preexec-like,
-   we parse out the name of the old DEBUG hook, then make a new function which
-   calls both that function and our starship hooks. We don't do this for
-   PROMPT_COMMAND because that would probably result in two prompts.
-
-   We need to quote the output of `$(jobs -p | wc -l)` since MacOS `wc` leaves
-   giant spaces in front of the number (e.g. "    3"), which messes up the
-   word-splitting. Instead, quote the whole thing, then let Rust do the whitespace
-   trimming within the jobs module
+/* GENERAL INIT SCRIPT NOTES
+
+Each init script will be passed as-is. Global notes for init scripts are in this
+comment, with additional per-script comments in the strings themselves.
+
+JOBS: The argument to `--jobs` is quoted because MacOS's `wc` leaves whitespace
+in the output. We pass it to starship and do the whitespace removal in Rust,
+to avoid the cost of an additional shell fork every shell draw.
 */
 
-/*
-Note to programmers: this and the zsh init will be evaluated on a single line.
-Use semicolons, avoid comments, and generally think like all newlines will be
-deleted.
+/* BASH INIT SCRIPT
+
+We use PROMPT_COMMAND and the DEBUG trap to generate timing information. We try
+to avoid clobbering what we can, and try to give the user ways around our
+clobbers, if it's unavoidable.
+
+A bash quirk is that the DEBUG trap is fired every time a command runs, even
+if it's later on in the pipeline. If uncorrected, this could cause bad timing
+data for commands like `slow | slow | fast`, since the timer starts at the start
+of the "fast" command.
+
+To solve this, we set a flag `PREEXEC_READY` when the prompt is drawn, and only
+start the timer if this flag is present. That way, timing is for the entire command,
+and not just a portion of it
 */
+
 const BASH_INIT: &str = r##"
+# Will be run before *every* command (even ones in pipes!)
 starship_preexec() {
+    # Avoid restarting the timer for commands in the same pipeline
     if [ "$PREEXEC_READY" = "true" ]; then
-        PREEXEC_READY=false;
-        STARSHIP_START_TIME=$(date +%s);
+        PREEXEC_READY=false
+        STARSHIP_START_TIME=$(date +%s)
     fi
-};
+}
+# Will be run before the prompt is drawn
 starship_precmd() {
-    STATUS=$?;
-    export STARSHIP_SHELL="bash";
-    "${starship_precmd_user_func-:}";
+    # Save the status, because commands in this pipeline will change $?
+    STATUS=$?
+
+    # Run the bash precmd function, if it's set. If not set, evaluates to no-op
+    "${starship_precmd_user_func-:}"
+
+    # Prepare the timer data, if needed.
     if [[ $STARSHIP_START_TIME ]]; then
-        STARSHIP_END_TIME=$(date +%s);
-        STARSHIP_DURATION=$((STARSHIP_END_TIME - STARSHIP_START_TIME));
-        PS1="$(starship prompt --status=$STATUS --jobs="$(jobs -p | wc -l)" --cmd-duration=$STARSHIP_DURATION)";
-        unset STARSHIP_START_TIME;
+        STARSHIP_END_TIME=$(date +%s)
+        STARSHIP_DURATION=$((STARSHIP_END_TIME - STARSHIP_START_TIME))
+        PS1="$(starship prompt --status=$STATUS --jobs="$(jobs -p | wc -l)" --cmd-duration=$STARSHIP_DURATION)"
+        unset STARSHIP_START_TIME
     else
-        PS1="$(starship prompt --status=$STATUS --jobs="$(jobs -p | wc -l)")";
-    fi;
-    PREEXEC_READY=true;
-};
+        PS1="$(starship prompt --status=$STATUS --jobs="$(jobs -p | wc -l)")"
+    fi
+    PREEXEC_READY=true;  # Signal that we can safely restart the timer
+}
+
+# If the user appears to be using https://github.com/rcaloras/bash-preexec,
+# then hook our functions into their framework.
 if [[ $preexec_functions ]]; then
-    preexec_functions+=(starship_preexec);
-    precmd_functions+=(starship_precmd);
-    STARSHIP_START_TIME=$(date +%s);
+    preexec_functions+=(starship_preexec)
+    precmd_functions+=(starship_precmd)
 else
-    dbg_trap="$(trap -p DEBUG | cut -d' ' -f3 | tr -d \')";
+# We want to avoid destroying an existing DEBUG hook. If we detect one, create 
+# a new function that runs both the existing function AND our function, then 
+# re-trap DEBUG to use this new function. This prevents a trap clobber.
+    dbg_trap="$(trap -p DEBUG | cut -d' ' -f3 | tr -d \')"
     if [[ -z "$dbg_trap" ]]; then
-        trap starship_preexec DEBUG;
+        trap starship_preexec DEBUG
     elif [[ "$dbg_trap" != "starship_preexec" && "$dbg_trap" != "starship_preexec_all" ]]; then
         function starship_preexec_all(){
-            $dbg_trap; starship_preexec;
-        };
-        trap starship_preexec_all DEBUG;
-    fi;
-    PROMPT_COMMAND=starship_precmd;
-    STARSHIP_START_TIME=$(date +%s);
-fi;
-"##;
+            $dbg_trap; starship_preexec
+        }
+        trap starship_preexec_all DEBUG
+    fi
 
-/* For zsh: preexec_functions and precmd_functions provide preexec/precmd in a
-   way that lets us avoid clobbering them.
+    # Finally, prepare the precmd function and set up the start time.
+    PROMPT_COMMAND=starship_precmd
+fi
 
-   Zsh quirk: preexec() is only fired if a command is actually run (unlike in
-   bash, where spamming empty commands still triggers DEBUG). This means a user
-   spamming ENTER at an empty command line will see increasing runtime (since
-   preexec never actually fires to reset the start time).
+# Set up the start time and STARSHIP_SHELL, which controls shell-specific sequences
+STARSHIP_START_TIME=$(date +%s)
+export STARSHIP_SHELL="bash"
+"##;
 
-   To fix this, only pass the time if STARSHIP_START_TIME is defined, and unset
-   it after passing the time, so that we only measure actual commands.
+/* ZSH INIT SCRIPT
 
-   We need to quote the output of the jobs command for the same reason as
-   bash.
+ZSH has a quirk where `preexec` is only run if a command is actually run (i.e
+pressing ENTER at an empty command line will not cause preexec to fire). This
+can cause timing issues, as a user who presses "ENTER" without running a command
+will see the time to the start of the last command, which may be very large.
+
+To fix this, we create STARSHIP_START_TIME upon preexec() firing, and destroy it
+after drawing the prompt. This ensures that the timing for one command is only
+ever drawn once (for the prompt immediately after it is run).
 */
 
 const ZSH_INIT: &str = r##"
+# Will be run before every prompt draw
 starship_precmd() {
-    STATUS=$?;
-    export STARSHIP_SHELL="zsh";
+    # Save the status, because commands in this pipeline will change $?
+    STATUS=$?
+
+    # Compute cmd_duration, if we have a time to consume
     if [[ $STARSHIP_START_TIME ]]; then
-        STARSHIP_END_TIME="$(date +%s)";
-        STARSHIP_DURATION=$((STARSHIP_END_TIME - STARSHIP_START_TIME));
-        PROMPT="$(starship prompt --status=$STATUS --cmd-duration=$STARSHIP_DURATION --jobs="$(jobs | wc -l)")";
-        unset STARSHIP_START_TIME;
+        STARSHIP_END_TIME="$(date +%s)"
+        STARSHIP_DURATION=$((STARSHIP_END_TIME - STARSHIP_START_TIME))
+        PROMPT="$(starship prompt --status=$STATUS --cmd-duration=$STARSHIP_DURATION --jobs="$(jobs | wc -l)")"
+        unset STARSHIP_START_TIME
     else
-        PROMPT="$(starship prompt --status=$STATUS --jobs="$(jobs | wc -l)")";
+        PROMPT="$(starship prompt --status=$STATUS --jobs="$(jobs | wc -l)")"
     fi
-};
+}
 starship_preexec(){
     STARSHIP_START_TIME="$(date +%s)"
-};
-if [[ -z "${precmd_functions+1}" ]]; then
-    precmd_functions=()
-fi;
-if [[ -z "${preexec_functions+1}" ]]; then
-    preexec_functions=()
-fi;
+}
+
+# If precmd/preexec arrays are not already set, set them. If we don't do this,
+# the code to detect whether starship_precmd is already in precmd_functions will
+# fail because the array doesn't exist (and same for starship_preexec)
+[[ -z "${precmd_functions+1}" ]] && precmd_functions=()
+[[ -z "${preexec_functions+1}" ]] && preexec_functions=()
+
+# If starship precmd/preexec functions are already hooked, don't double-hook them
+# to avoid unnecessary performance degradation in nested shells
 if [[ ${precmd_functions[(ie)starship_precmd]} -gt ${#precmd_functions} ]]; then
-    precmd_functions+=(starship_precmd);
-fi;
+    precmd_functions+=(starship_precmd)
+fi
 if [[ ${preexec_functions[(ie)starship_preexec]} -gt ${#preexec_functions} ]]; then
-    preexec_functions+=(starship_preexec);
-fi;
-STARSHIP_START_TIME="$(date +%s)";
+    preexec_functions+=(starship_preexec)
+fi
+# Set up a function to redraw the prompt if the user switches vi modes
 function zle-keymap-select
 {
-    PROMPT=$(starship prompt --keymap=$KEYMAP --jobs="$(jobs | wc -l)");
-    zle reset-prompt;
-};
-zle -N zle-keymap-select;
+    PROMPT=$(starship prompt --keymap=$KEYMAP --jobs="$(jobs | wc -l)")
+    zle reset-prompt
+}
+
+STARSHIP_START_TIME="$(date +%s)"
+zle -N zle-keymap-select
+export STARSHIP_SHELL="zsh"
 "##;
 
-/* Fish setup is simple because they give us CMD_DURATION. Just account for name
-changes between 2.7/3.0 and do some math to convert ms->s and we can use it */
 const FISH_INIT: &str = r##"
-function fish_prompt;
-    set -l exit_code $status;
-    set -l CMD_DURATION "$CMD_DURATION$cmd_duration";
-    set -l starship_duration (math --scale=0 "$CMD_DURATION / 1000");
-    starship prompt --status=$exit_code --cmd-duration=$starship_duration --jobs=(count (jobs -p));
-end;
+function fish_prompt
+    set -l exit_code $status
+    # Account for changes in variable name between v2.7 and v3.0
+    set -l CMD_DURATION "$CMD_DURATION$cmd_duration"
+    set -l starship_duration (math --scale=0 "$CMD_DURATION / 1000")
+    starship prompt --status=$exit_code --cmd-duration=$starship_duration --jobs=(count (jobs -p))
+end
 "##;
diff --git a/src/main.rs b/src/main.rs
index 523b2d9bc..7143a59bf 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -58,6 +58,10 @@ fn main() {
         .help("The number of currently running jobs")
         .takes_value(true);
 
+    let init_scripts_arg = Arg::with_name("print_full_init")
+        .long("print-full-init")
+        .help("Print the main initialization script (as opposed to the init stub)");
+
     let matches = App::new("starship")
         .about("The cross-shell prompt for astronauts. ☄🌌️")
         // pull the version number from Cargo.toml
@@ -69,7 +73,8 @@ fn main() {
         .subcommand(
             SubCommand::with_name("init")
                 .about("Prints the shell function used to execute starship")
-                .arg(&shell_arg),
+                .arg(&shell_arg)
+                .arg(&init_scripts_arg),
         )
         .subcommand(
             SubCommand::with_name("prompt")
@@ -99,7 +104,11 @@ fn main() {
     match matches.subcommand() {
         ("init", Some(sub_m)) => {
             let shell_name = sub_m.value_of("shell").expect("Shell name missing.");
-            init::init(shell_name)
+            if sub_m.is_present("print_full_init") {
+                init::init_main(shell_name);
+            } else {
+                init::init_stub(shell_name);
+            }
         }
         ("prompt", Some(sub_m)) => print::prompt(sub_m.clone()),
         ("module", Some(sub_m)) => {