Add ability to inherit an env var on the CLI (#6746)

In addition to Wasmtime's preexisting support for `--env FOO=bar` this
commit additionally adds support for `--env FOO` which is inspired by
Docker to inherit the environment variable `FOO` from the calling
process as a shortcut for `--env FOO=$FOO`.

src/commands/run.rs

@@ -51,12 +51,12 @@ fn parse_module(s: OsString) -> anyhow::Result<PathBuf> {
     }
 }
 
-fn parse_env_var(s: &str) -> Result<(String, String)> {
-    let parts: Vec<_> = s.splitn(2, '=').collect();
-    if parts.len() != 2 {
-        bail!("must be of the form `key=value`");
-    }
-    Ok((parts[0].to_owned(), parts[1].to_owned()))
+fn parse_env_var(s: &str) -> Result<(String, Option<String>)> {
+    let mut parts = s.splitn(2, '=');
+    Ok((
+        parts.next().unwrap().to_string(),
+        parts.next().map(|s| s.to_string()),
+    ))
 }
 
 fn parse_map_dirs(s: &str) -> Result<(String, String)> {
@@ -156,9 +156,15 @@ pub struct RunCommand {
     #[clap(long = "dir", number_of_values = 1, value_name = "DIRECTORY")]
     dirs: Vec<String>,
 
-    /// Pass an environment variable to the program
-    #[clap(long = "env", number_of_values = 1, value_name = "NAME=VAL", value_parser = parse_env_var)]
-    vars: Vec<(String, String)>,
+    /// Pass an environment variable to the program.
+    ///
+    /// The `--env FOO=BAR` form will set the environment variable named `FOO`
+    /// to the value `BAR` for the guest program using WASI. The `--env FOO`
+    /// form will set the environment variable named `FOO` to the same value it
+    /// has in the calling process for the guest, or in other words it will
+    /// cause the environment variable `FOO` to be inherited.
+    #[clap(long = "env", number_of_values = 1, value_name = "NAME[=VAL]", value_parser = parse_env_var)]
+    vars: Vec<(String, Option<String>)>,
 
     /// The name of the function to run
     #[clap(long, value_name = "FUNCTION")]
@@ -689,7 +695,7 @@ fn populate_with_wasi(
     module: Module,
     preopen_dirs: Vec<(String, Dir)>,
     argv: &[String],
-    vars: &[(String, String)],
+    vars: &[(String, Option<String>)],
     wasi_modules: &WasiModules,
     listenfd: bool,
     mut tcplisten: Vec<TcpListener>,
@@ -698,7 +704,16 @@ fn populate_with_wasi(
         wasmtime_wasi::add_to_linker(linker, |host| host.wasi.as_mut().unwrap())?;
 
         let mut builder = WasiCtxBuilder::new();
-        builder = builder.inherit_stdio().args(argv)?.envs(vars)?;
+        builder = builder.inherit_stdio().args(argv)?;
+
+        for (key, value) in vars {
+            let value = match value {
+                Some(value) => value.clone(),
+                None => std::env::var(key)
+                    .map_err(|_| anyhow!("environment varialbe `{key}` not found"))?,
+            };
+            builder = builder.env(key, &value)?;
+        }
 
         let mut num_fd: usize = 3;
 

tests/all/cli_tests.rs

@@ -484,6 +484,50 @@ fn hello_wasi_snapshot0_from_stdin() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn specify_env() -> Result<()> {
+    // By default no env is inherited
+    let output = get_wasmtime_command()?
+        .args(&["run", "tests/all/cli_tests/print_env.wat"])
+        .env("THIS_WILL_NOT", "show up in the output")
+        .output()?;
+    assert!(output.status.success());
+    assert_eq!(String::from_utf8_lossy(&output.stdout), "");
+
+    // Specify a single env var
+    let output = get_wasmtime_command()?
+        .args(&[
+            "run",
+            "--env",
+            "FOO=bar",
+            "tests/all/cli_tests/print_env.wat",
+        ])
+        .output()?;
+    assert!(output.status.success());
+    assert_eq!(String::from_utf8_lossy(&output.stdout), "FOO=bar\n");
+
+    // Inherit a single env var
+    let output = get_wasmtime_command()?
+        .args(&["run", "--env", "FOO", "tests/all/cli_tests/print_env.wat"])
+        .env("FOO", "bar")
+        .output()?;
+    assert!(output.status.success());
+    assert_eq!(String::from_utf8_lossy(&output.stdout), "FOO=bar\n");
+
+    // Inherit a nonexistent env var
+    let output = get_wasmtime_command()?
+        .args(&[
+            "run",
+            "--env",
+            "SURELY_THIS_ENV_VAR_DOES_NOT_EXIST_ANYWHERE_RIGHT",
+            "tests/all/cli_tests/print_env.wat",
+        ])
+        .output()?;
+    assert!(!output.status.success());
+
+    Ok(())
+}
+
 #[cfg(unix)]
 #[test]
 fn run_cwasm_from_stdin() -> Result<()> {

tests/all/cli_tests/print_env.wat

@@ -0,0 +1,76 @@
+(module
+  (import "wasi_snapshot_preview1" "fd_write"
+    (func $fd_write (param i32 i32 i32 i32) (result i32)))
+
+  (import "wasi_snapshot_preview1" "environ_get"
+    (func $environ_get (param i32 i32) (result i32)))
+
+  (memory (export "memory") 1)
+
+  (func (export "_start")
+    (local $envptrs i32)
+    (local $envmem i32)
+    (local $env i32)
+
+    (local.set $envptrs (i32.mul (memory.grow (i32.const 1)) (i32.const 65536)))
+    (local.set $envmem (i32.mul (memory.grow (i32.const 1)) (i32.const 65536)))
+
+    (if (i32.ne
+          (call $environ_get (local.get $envptrs) (local.get $envmem))
+          (i32.const 0))
+        (unreachable))
+
+    (loop
+      (local.set $env (i32.load (local.get $envptrs)))
+      (local.set $envptrs (i32.add (local.get $envptrs) (i32.const 4)))
+      (if (i32.eq (local.get $env) (i32.const 0)) (return))
+
+      (call $write_all (local.get $env) (call $strlen (local.get $env)))
+      (call $write_all (i32.const 10) (i32.const 1))
+      br 0
+    )
+  )
+
+  (func $write_all (param $ptr i32) (param $len i32)
+    (local $rc i32)
+    (local $iov i32)
+    (local $written i32)
+
+    (local.set $written (i32.const 80))
+    (local.set $iov (i32.const 100))
+
+    (loop
+      (local.get $len)
+      if
+        (i32.store (local.get $iov) (local.get $ptr))
+        (i32.store offset=4 (local.get $iov) (local.get $len))
+        (local.set $rc
+          (call $fd_write
+            (i32.const 1)
+            (local.get $iov)
+            (i32.const 1)
+            (local.get $written)))
+        (if (i32.ne (local.get $rc) (i32.const 0)) (unreachable))
+
+        (local.set $len (i32.sub (local.get $len) (i32.load (local.get $written))))
+        (local.set $ptr (i32.add (local.get $ptr) (i32.load (local.get $written))))
+      end
+    )
+  )
+
+  (func $strlen (param $ptr i32) (result i32)
+    (local $len i32)
+    (loop
+      (i32.load8_u (i32.add (local.get $ptr) (local.get $len)))
+      if
+        (local.set $len (i32.add (local.get $len) (i32.const 1)))
+        br 1
+      end
+    )
+    local.get $len
+  )
+
+  (data (i32.const 10) "\n")
+)
+
+