sysusers: Make loading /etc/passwd optional

On general principle; this fixes a test failure in the lint unit
tests which don't create /etc/passwd.

But also, someone might reasonably create an image this way.

Signed-off-by: Colin Walters <walters@verbum.org>

sysusers/src/lib.rs

@@ -201,7 +201,7 @@ pub fn read_sysusers(rootfs: &Dir) -> Result<Vec<SysusersEntry>> {
 }
 
 /// The result of analyzing /etc/{passwd,group} in a root vs systemd-sysusers.
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct SysusersAnalysis {
     /// Entries which are found in /etc/passwd but not present in systemd-sysusers.
     pub missing_users: BTreeSet<String>,
@@ -230,8 +230,14 @@ pub fn analyze(rootfs: &Dir) -> Result<SysusersAnalysis> {
         id: Option<u32>,
     }
 
-    let mut passwd = nameservice::passwd::load_etc_passwd(rootfs)
+    let Some(passwd) = nameservice::passwd::load_etc_passwd(rootfs)
         .map_err(|e| Error::PasswdLoadFailure(e.to_string()))?
+    else {
+        // If there's no /etc/passwd then we're done
+        return Ok(SysusersAnalysis::default());
+    };
+
+    let mut passwd = passwd
         .into_iter()
         .map(|mut e| {
             // Make the name be the map key, leaving the old value a stub

sysusers/src/nameservice/passwd.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
 use anyhow::{anyhow, Context, Result};
-use cap_std_ext::cap_std::fs::Dir;
+use cap_std_ext::{cap_std::fs::Dir, dirext::CapStdExtDirExt};
 use std::io::{BufRead, BufReader, Write};
 
 // Entry from passwd file.
@@ -78,9 +78,12 @@ pub(crate) fn parse_passwd_content(content: impl BufRead) -> Result<Vec<PasswdEn
     Ok(passwds)
 }
 
-pub(crate) fn load_etc_passwd(rootfs: &Dir) -> Result<Vec<PasswdEntry>> {
-    let r = rootfs.open("etc/passwd").map(BufReader::new)?;
-    parse_passwd_content(r)
+pub(crate) fn load_etc_passwd(rootfs: &Dir) -> Result<Option<Vec<PasswdEntry>>> {
+    if let Some(r) = rootfs.open_optional("etc/passwd")? {
+        parse_passwd_content(BufReader::new(r)).map(Some)
+    } else {
+        Ok(None)
+    }
 }
 
 #[cfg(test)]