build: check stat's exit status and honor DPKG_ORIGINS_DIR

hashes_in_context never checked stat's exit status and parsed its size
with unwrap_or(0), silently recording zero-size artifacts in the
generated .changes/.buildinfo; stat failures and unparsable sizes are
now errors naming the file. current_vendor hardcoded
/etc/dpkg/origins/default while dpkg honors DPKG_ORIGINS_DIR (already
in the file's own ENV_ALLOWED list); the origins default is now
resolved against it with the usual fallback.

src/build/env.rs

@@ -3,7 +3,7 @@
 //! sanitized environment recorded in `.buildinfo` files.
 
 use std::collections::BTreeMap;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 /// Number of parallel jobs to advertise in `DEB_BUILD_OPTIONS`.
 pub fn num_parallel() -> usize {
@@ -57,15 +57,31 @@ pub fn arch_env(host_arch: Option<&str>) -> Result<BTreeMap<String, String>, Str
     crate::debian::arch::arch_env(host_arch)
 }
 
-/// Read the current vendor name from `/etc/dpkg/origins/default`
-/// (its `Vendor:` or `Origin:` field), defaulting to `"debian"`.
+/// Read the current vendor name from the active dpkg origins `default` file
+/// (`$DPKG_ORIGINS_DIR/default`, falling back to `/etc/dpkg/origins/default`;
+/// its `Vendor:` or `Origin:` field), defaulting to `"debian"`.
 pub fn current_vendor() -> String {
-    std::fs::read_to_string(Path::new("/etc/dpkg/origins/default"))
+    let path = resolve_origins_default(
+        std::env::var("DPKG_ORIGINS_DIR").ok().as_deref(),
+        "/etc/dpkg/origins",
+    );
+    std::fs::read_to_string(path)
         .ok()
         .and_then(|content| vendor_from_origins_content(&content))
         .unwrap_or_else(|| "debian".to_string())
 }
 
+/// Resolve the path of the active dpkg origins file from the
+/// `DPKG_ORIGINS_DIR` value (the directory holding the origin files, where
+/// `default` selects the active one) and the fallback directory
+/// (`/etc/dpkg/origins`). An unset or empty directory value falls back.
+fn resolve_origins_default(origins_dir: Option<&str>, fallback_dir: &str) -> PathBuf {
+    let dir = origins_dir
+        .filter(|d| !d.is_empty())
+        .unwrap_or(fallback_dir);
+    Path::new(dir).join("default")
+}
+
 /// Extract the vendor name from the content of a dpkg origins file: its
 /// `Vendor:` field, falling back to `Origin:` when absent. `None` when
 /// neither field carries a non-empty value.
@@ -306,6 +322,26 @@ mod tests {
         assert_eq!(vendor_from_origins_content("Suite: stable\n"), None);
     }
 
+    /// `current_vendor` must honor `DPKG_ORIGINS_DIR` (already on the
+    /// `.buildinfo` allow-list) when locating the `default` origins file,
+    /// falling back to `/etc/dpkg/origins/default` when unset or empty.
+    #[test]
+    fn origins_default_path_honors_dpkg_origins_dir() {
+        assert_eq!(
+            resolve_origins_default(Some("/custom/origins"), "/etc/dpkg/origins"),
+            PathBuf::from("/custom/origins/default")
+        );
+        assert_eq!(
+            resolve_origins_default(None, "/etc/dpkg/origins"),
+            PathBuf::from("/etc/dpkg/origins/default")
+        );
+        // An empty value behaves as unset, like dpkg's `$dir || $default`.
+        assert_eq!(
+            resolve_origins_default(Some(""), "/etc/dpkg/origins"),
+            PathBuf::from("/etc/dpkg/origins/default")
+        );
+    }
+
     #[test]
     fn environment_escaping() {
         // The function reads the process env; just verify formatting helpers