More robust shell identification logic (#67)

All silver subcommands look up the parent process’s name, and use
that, verbatim, to identify the shell in use.  This has several
problems:

 - On Windows, the parent process’s name may or may not end with
   “.exe”; there was special case logic to handle “powershell.exe”
   versus “powershell” but not “bash.exe” versus “bash”.  Relatedly,
   the parent process’s name ought to be interpreted case-
   insensitively on Windows; BASH.EXE is the same as bash.exe.

 - On Unix, there is a notion of a _login_ shell (invoked directly by
   login(1) for console logins, or sshd(1) for remote logins); these
   are identified by a leading dash in their process name, *not* by a
   special option.  There was no code to handle this at all (bug #67).

 - sysinfo is slow, because it reads a whole bunch of files in
   /proc whether it’s going to need the data within or not.
   Some but not all of this overhead can be eliminated by using
   `System::new_with_specifics(RefreshKind::new())` instead of
   `System::new_all()`, but even then the wall-clock time for
   `silver init` drops from 0.021s to 0.002s (release build)
   if sysinfo is not invoked at all.

This patch addresses all these problems, as follows:

 - There is a new command line option `--shell` which may be used to
   explicitly set the shell in use.  Also, support for the
   `SILVER_SHELL` environment variable (used for this purpose in 1.1
   and previous) is restored.  sysinfo is only consulted if neither
   the command line option, nor the environment variable, is
   available.

 - No matter where we get the process name from, it is lowercased,
   a trailing “.exe” is removed, a leading dash is removed, and any
   leading directory names are also removed (bug #16).

 - The initialization scripts set `SILVER_SHELL`, so sysinfo will be
   consulted at most once per session, not once per command.

I also reorganized the code in the Command::Init branch of main for
readability’s sake, and made the “unknown shell” error message not be
a panic.  (Panics are for _bugs_, not for errors with a cause outside
the program.)

src/cli.rs

@@ -9,6 +9,8 @@ pub use structopt::StructOpt;
 pub struct Silver {
     #[structopt(short, long)]
     pub config: Option<String>,
+    #[structopt(short, long)]
+    pub shell: Option<String>,
     #[structopt(subcommand)]
     pub cmd:    Command,
 }

src/init.bash

@@ -2,4 +2,5 @@ PROMPT_COMMAND=silver_prompt
 silver_prompt() {
     PS1="$(code=$? jobs=$(jobs -p | wc -l) silver lprint)"
 }
+export SILVER_SHELL="@SILVER_SHELL@"
 export VIRTUAL_ENV_DISABLE_PROMPT=1

src/init.ion

@@ -1,4 +1,5 @@
 fn PROMPT
   env code=$? jobs=$(jobs -p | wc -l) silver lprint
 end
-export VIRTUAL_ENV_DISABLE_PROMPT = 1 # Doesn't make any sense yet
+export SILVER_SHELL = "@SILVER_SHELL@"
+export VIRTUAL_ENV_DISABLE_PROMPT = 1

src/init.ps1

@@ -6,4 +6,5 @@ function prompt {
     Start-Process -Wait -NoNewWindow silver lprint
     "$([char]0x1b)[0m"
 }
-$Env:VIRTUAL_ENV_DISABLE_PROMPT = 1
+$env:SILVER_SHELL = "@SILVER_SHELL@"
+$env:VIRTUAL_ENV_DISABLE_PROMPT = 1

src/main.rs

@@ -7,8 +7,9 @@ mod sh;
 
 use cli::*;
 use once_cell::sync::{Lazy, OnceCell};
+use std::env;
 use std::path::{Path, PathBuf};
-use sysinfo::{get_current_pid, ProcessExt, System, SystemExt};
+use sysinfo::{get_current_pid, ProcessExt, RefreshKind, System, SystemExt};
 
 static CONFIG_PATH: OnceCell<PathBuf> = OnceCell::new();
 
@@ -21,6 +22,10 @@ static CONFIG: Lazy<config::Config> = Lazy::new(|| {
     .expect("Failed to read config")
 });
 
+const INIT_BASH: &str = include_str!("init.bash");
+const INIT_PS1: &str = include_str!("init.ps1");
+const INIT_ION: &str = include_str!("init.ion");
+
 #[derive(Clone, Debug)]
 pub struct Segment {
     background: String,
@@ -38,47 +43,110 @@ impl Default for Segment {
     }
 }
 
-fn main() {
-    let sys = System::new_all();
-    let process = sys.get_process(get_current_pid().unwrap()).unwrap();
-    let parent = sys.get_process(process.parent().unwrap()).unwrap();
-    let shell = parent.name().trim();
+/// Helper function for trimming a String in place. Derived from
+/// https://users.rust-lang.org/t/trim-string-in-place/15809/9
+fn replace_with_subslice<F>(this: &mut String, f: F)
+    where F: FnOnce(&str) -> &str
+{
+    let original_len = this.len();
+    let new_slice: &str = f(this);
+    let start = (new_slice.as_ptr() as usize)
+        .wrapping_sub(this.as_ptr() as usize);
+    if original_len < start {
+        this.clear();
+        return;
+    }
+
+    let len = new_slice.len();
+    if start != 0 {
+        this.drain(..start);
+    }
+    this.truncate(len);
+}
+
+/// Identify the type of shell in use.
+fn get_shell(opt: &cli::Silver) -> String {
+    let mut shell: String =
+        if let Some(ref s) = opt.shell {
+            // If --shell was given on the command line, use that.
+            s.clone()
+        } else if let Ok(s) = env::var("SILVER_SHELL") {
+            // For backward compatibility with 1.1 and earlier,
+            // use the value of the SILVER_SHELL environment variable.
+            s
+        } else {
+            // Use the name of the parent process, if we can.
+            // Minimize the amount of information loaded by sysinfo.
+            // FIXME: Proper error handling, not all these unwraps.
+            let mut sys = System::new_with_specifics(RefreshKind::new());
+            // It'd be nice if either the stdlib or sysinfo exposed
+            // getppid() directly, but they don't.
+            let mypid = get_current_pid().unwrap();
+            sys.refresh_process(mypid);
+            let parentpid = sys.get_process(mypid).unwrap().parent().unwrap();
+            sys.refresh_process(parentpid);
 
+            sys.get_process(parentpid).unwrap().name().to_string()
+        };
+
+    // For Windows compatibility, lowercase the shell's name
+    // ("BASH.EXE" is the same as "bash.exe").
+    shell.make_ascii_lowercase();
+
+    // Remove any leading or trailing whitespace from `shell`.
+    // Remove anything up to and including the last '/' or '\\',
+    //   in case the shell was identified by absolute path.
+    // Remove a trailing ".exe" if present, for Windows compatibility.
+    // Remove a leading "-" if present; on Unix this is a flag indicating
+    //   a login shell (see login(1) and sh(1)), not part of the name.
+    // These are done unconditionally, not just when we are using
+    // the name of the parent process to identify the shell, because
+    // the installation instructions for older versions of silver
+    // said to set SILVER_SHELL to "$0" without trimming anything
+    // from it.
+    replace_with_subslice(&mut shell, |s| {
+        let s = s.trim();
+        let s = s.rsplit(&['/', '\\'][..]).next().unwrap_or(s);
+        let s = s.strip_prefix('-').unwrap_or(s);
+        let s = s.strip_suffix(".exe").unwrap_or(s);
+        s
+    });
+
+    shell
+}
+
+fn main() {
     let opt = cli::Silver::from_args();
 
-    if let Some(path) = opt.config {
+    if let Some(ref path) = opt.config {
         let path = Path::new(path.as_str()).canonicalize().unwrap();
         CONFIG_PATH.set(path).unwrap()
     }
 
+    let _shell = get_shell(&opt);
+    let shell = _shell.as_str();
+
     match opt.cmd {
         Command::Init => {
-            print!(
-                "{}",
-                match shell {
-                    "bash" => include_str!("init.bash"),
-                    "powershell" | "pwsh" | "powershell.exe" | "pwsh.exe" =>
-                        include_str!("init.ps1"),
-                    "ion" => include_str!("init.ion"),
-                    _ =>
-                        panic!(
-                            "unknown shell: \"{}\". Supported shells: bash, ion, powershell",
-                            shell
-                        ),
+            let script = match shell {
+                "bash" => INIT_BASH,
+                "powershell" | "pwsh" => INIT_PS1,
+                "ion" => INIT_ION,
+                _ => {
+                    use std::process::exit;
+                    eprintln!("silver: unknown shell: \"{}\".", shell);
+                    eprintln!("silver: supported shells: bash, ion, powershell");
+                    exit(1);
                 }
-                .replace(
-                    "silver",
-                    format!(
-                        "silver{}",
-                        if let Some(path) = CONFIG_PATH.get() {
-                            format!(" --config {}", path.display())
-                        } else {
-                            String::new()
-                        }
-                    )
-                    .as_str()
-                )
-            )
+            };
+            let script = script.replace("@SILVER_SHELL@", shell);
+            let script = if let Some(path) = CONFIG_PATH.get() {
+                script.replace("silver", format!("silver --config {}",
+                                                 path.display()).as_str())
+            } else {
+                script
+            };
+            print!("{}", script);
         }
         Command::Lprint => {
             print::prompt(&shell, &CONFIG.left, |_, (_, c, n)| {