deps: treat corrupt Provides as undecidable, not unmet

A versioned Provides whose version failed to parse was silently
skipped, so a corrupt dpkg status entry could yield a wrong 'unmet'
verdict where the truth is 'cannot decide': unparseable provided
versions now set lackinfos like unparseable installed versions do.
Provides alternatives with a non-= constraint are likewise rejected as
a whole field (dpkg rejects the entry), replacing the skip-per-
alternative behavior that contradicted the code's own comment.

src/debian/deps.rs

@@ -10,7 +10,7 @@
 //! (<https://manpages.debian.org/libdpkg-perl>) and were validated
 //! differentially against the real tool.
 
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
 use std::sync::OnceLock;
 
@@ -583,6 +583,24 @@ pub struct Facts {
     build_arch: String,
     installed: HashMap<String, Vec<InstalledPkg>>,
     provided: HashMap<String, Vec<ProvidedPkg>>,
+    /// Virtual packages whose `Provides` entries had to be rejected as a
+    /// whole (unparseable field, malformed version, or a relation other
+    /// than `=`), mirroring a dpkg rejection of the entry: versioned
+    /// relations on them stay undecidable instead of turning unmet.
+    unreadable_provides: HashSet<String>,
+}
+
+/// Best-effort virtual package names mentioned in a raw `Provides` value,
+/// used to remember rejected entries whose content cannot even be parsed
+/// (the full dependency grammar is deliberately not re-applied here).
+fn raw_provides_names(field: &str) -> impl Iterator<Item = &str> {
+    field
+        .split([',', '|'])
+        .filter_map(|alt| alt.split_whitespace().next())
+        .map(|tok| match tok.split_once(':') {
+            Some((name, _qual)) => name,
+            None => tok,
+        })
 }
 
 impl Facts {
@@ -593,6 +611,7 @@ impl Facts {
             build_arch: build_arch.to_string(),
             installed: HashMap::new(),
             provided: HashMap::new(),
+            unreadable_provides: HashSet::new(),
         }
     }
 
@@ -609,6 +628,11 @@ impl Facts {
     }
 
     /// Record that `provider` provides the virtual package `virtual_name`.
+    ///
+    /// A `relation` other than [`Relation::Eq`], or a `version` that fails
+    /// to parse, makes the provide unusable: versioned relations on
+    /// `virtual_name` then evaluate to `None` (undecidable) instead of a
+    /// possibly-wrong `Some(false)`.
     pub fn add_provided(
         &mut self,
         virtual_name: &str,
@@ -639,7 +663,10 @@ impl Facts {
     /// Only stanzas whose `Status` ends with `ok installed` participate;
     /// their `Provides` field registers versioned/unversioned virtual
     /// packages (architecture-restricted provides are reduced against
-    /// `host_arch`).
+    /// `host_arch`). A `Provides` field that fails to parse, or that
+    /// carries a relation other than `=`, is rejected as a whole (like
+    /// dpkg rejects the entry) and its virtual names are remembered as
+    /// unreadable.
     pub fn from_status(content: &str, host_arch: &str, build_arch: &str) -> Facts {
         let mut facts = Facts::new(host_arch, build_arch);
         for para in crate::debian::control::parse_paragraphs(content) {
@@ -664,20 +691,38 @@ impl Facts {
                     union: true,
                     build_dep: false,
                 };
-                // Virtual (Provides) fields only accept '=' relations; a
-                // parse failure skips the whole field, like dpkg does.
-                let Ok(parsed) = Deps::parse_inner(provides, &opts, true) else {
+                // Virtual (Provides) fields only accept versionless or
+                // exactly '='-versioned alternatives; a field that fails
+                // to parse, or that carries any other relation, is
+                // rejected as a whole, like dpkg rejects the entry. The
+                // mentioned virtual names are remembered so versioned
+                // relations on them stay undecidable instead of turning
+                // into a possibly-wrong "unmet" verdict.
+                let mut parsed = None;
+                let mut rejected: Vec<String> = Vec::new();
+                match Deps::parse_inner(provides, &opts, true) {
+                    Ok(deps) => {
+                        if deps.clauses().flatten().any(|alt| {
+                            alt.constraint
+                                .as_ref()
+                                .is_some_and(|c| c.relation != Relation::Eq)
+                        }) {
+                            rejected
+                                .extend(deps.clauses().flatten().map(|alt| alt.package.clone()));
+                        } else {
+                            parsed = Some(deps);
+                        }
+                    }
+                    // The field could not even be parsed: recover the raw
+                    // names so the rejection is still remembered.
+                    Err(_) => rejected.extend(raw_provides_names(provides).map(str::to_string)),
+                }
+                facts.unreadable_provides.extend(rejected);
+                let Some(parsed) = parsed else {
                     continue;
                 };
                 for clause in parsed.clauses() {
                     for alt in clause {
-                        if alt
-                            .constraint
-                            .as_ref()
-                            .is_some_and(|c| c.relation != Relation::Eq)
-                        {
-                            continue;
-                        }
                         let provided_version = alt
                             .constraint
                             .as_ref()
@@ -755,11 +800,21 @@ impl Facts {
             }
         }
 
+        // A rejected Provides entry for this virtual package carries
+        // information that could not be read: a versioned relation can
+        // then not be decided (an unversioned one only needs the name,
+        // which the rejection removed).
+        if rel.constraint.is_some() && self.unreadable_provides.contains(&rel.package) {
+            lackinfos = true;
+        }
+
         if let Some(providers) = self.provided.get(&rel.package) {
             for vp in providers {
-                // Only unversioned provides and strictly-versioned provides
-                // can satisfy a dependency.
+                // Only unversioned provides and exactly-versioned provides
+                // can satisfy a dependency; anything else is an invalid
+                // provide and leaves the relation undecidable.
                 if vp.relation.is_some_and(|r| r != Relation::Eq) {
+                    lackinfos = true;
                     continue;
                 }
                 match &rel.constraint {
@@ -767,11 +822,16 @@ impl Facts {
                         let Some(vp_version) = &vp.version else {
                             continue;
                         };
-                        if let Ok(vp_v) = DebianVersion::parse(vp_version)
-                            && constraint.relation.eval(&vp_v, &constraint.version)
-                        {
+                        match DebianVersion::parse(vp_version) {
+                            Ok(vp_v) if constraint.relation.eval(&vp_v, &constraint.version) => {
                                 return Some(true);
                             }
+                            // An unreadable provided version, like an
+                            // unreadable installed version, makes the
+                            // relation undecidable instead of unmet.
+                            Err(_) => lackinfos = true,
+                            Ok(_) => {}
+                        }
                     }
                     None => return Some(true),
                 }
@@ -1251,6 +1311,73 @@ Provides: old-virtual (= 0.5)
         assert_eq!(facts.evaluate_relation(&o("old-virtual")), Some(true));
     }
 
+    /// A `Provides` entry that dpkg would reject as a whole (a malformed
+    /// provided version, or a relation other than `=`) must not degrade to
+    /// a bogus "unmet" verdict: versioned relations on the affected virtual
+    /// names stay undecidable.
+    #[test]
+    fn corrupt_provides_are_undecidable_not_unmet() {
+        // `not-a-version!` is rejected by DebianVersion::parse ('!' is not
+        // a legal version character), unlike dpkg-invalid but here-valid
+        // letter-only spellings.
+        let status = "\
+Package: bad-version-provider
+Status: install ok installed
+Version: 1.0
+Architecture: amd64
+Provides: virt (= not-a-version!)
+
+Package: bad-relation-provider
+Status: install ok installed
+Version: 1.0
+Architecture: amd64
+Provides: virt2 (>= 1.0), plain
+";
+        let facts = Facts::from_status(status, "amd64", "amd64");
+        let o = |s: &str| parse_simple(s, true).unwrap();
+
+        // The provided version fails to parse: undecidable, not unmet.
+        assert_eq!(facts.evaluate_relation(&o("virt (>= 0.5)")), None);
+        // A non-'=' relation invalidates the whole Provides field, so
+        // neither of its alternatives may produce a verdict.
+        assert_eq!(facts.evaluate_relation(&o("virt2 (>= 0.5)")), None);
+        assert_eq!(facts.evaluate_relation(&o("plain (>= 0.5)")), None);
+        // Unversioned relations only need the name, which the rejected
+        // entries no longer provide: genuinely unmet.
+        assert_eq!(facts.evaluate_relation(&o("virt")), Some(false));
+        assert_eq!(facts.evaluate_relation(&o("virt2")), Some(false));
+        assert_eq!(facts.evaluate_relation(&o("plain")), Some(false));
+    }
+
+    /// The same undecidable verdicts through the direct facts API: an
+    /// unreadable provided version and an invalid (non-`=`) provide each
+    /// leave a versioned relation undecided, while a readable provider
+    /// elsewhere still satisfies it.
+    #[test]
+    fn unreadable_provided_version_and_relation_are_undecidable() {
+        let o = |s: &str| parse_simple(s, true).unwrap();
+
+        let mut facts = Facts::new("amd64", "amd64");
+        facts.add_installed("provider", "1.0", "amd64", "no");
+        // `virt (= not-a-version!)`: the version string fails to parse.
+        facts.add_provided(
+            "virt",
+            Some(Relation::Eq),
+            Some("not-a-version!"),
+            "provider",
+        );
+        assert_eq!(facts.evaluate_relation(&o("virt (>= 0.5)")), None);
+        // `virt2 (>= 1.0)`: a non-'=' provide is invalid.
+        facts.add_provided("virt2", Some(Relation::Ge), Some("1.0"), "provider");
+        assert_eq!(facts.evaluate_relation(&o("virt2 (>= 0.5)")), None);
+
+        // A readable provider decides the relation when it matches;
+        // otherwise the unreadable entry keeps it undecidable.
+        facts.add_provided("virt", Some(Relation::Eq), Some("2.0"), "better");
+        assert_eq!(facts.evaluate_relation(&o("virt (>= 0.5)")), Some(true));
+        assert_eq!(facts.evaluate_relation(&o("virt (>> 2.0)")), None);
+    }
+
     #[test]
     fn simplify_reports_unmet() {
         let facts = Facts::from_status(STATUS, "amd64", "amd64");